( CNN ) -- Kofi Annan , whose initiative to forge peace in war-ravaged Syria failed to take hold , said he has resigned as the U.N. and Arab League joint special envoy because of `` increasing militarization on the ground '' and `` the clear lack of unity '' at the U.N. Security Council . Annan said he told U.N. Secretary-General Ban Ki-moon and Arab League Secretary-General Nabil el-Araby that he did n't want to renew his mandate when it expires August 31 . Syria state-run TV said the nation `` expresses regret for the resignation , '' and Ban said he and el-Araby are looking for a successor to take on the `` crucial peacemaking effort . '' In Geneva , Annan said he accepted the job despite its challenges , a task that some had dubbed `` Mission Impossible . '' He said it was a `` sacred duty '' to bring about peace , end the killings and abuse of civilians , and set a course toward political change . `` The severity of the humanitarian costs of the conflict , and the exceptional threats posed by this crisis to international peace and security , justified the attempts to secure a peaceful transition to a political settlement , however daunting the challenge . The increasing militarization on the ground and the clear lack of unity in the Security Council have fundamentally changed the circumstances for the effective exercise of my role , '' he said . Obama authorized covert support for Syrian rebels , sources say . `` Yet the bloodshed continues , most of all because of the Syrian government 's intransigence and continuing refusal to implement the six-point plan , and also because of the escalating military campaign of the opposition , all of which is compounded by the disunity of the international community . At a time when we need -- when the Syrian people desperately need action , there continues to be finger-pointing and name-calling in the Security Council . '' The nearly 17-month-long Syrian crisis started when the regime brutally cracked down on peaceful protesters in March 2011 and morphed into a nationwide uprising . Among the five permanent members of the council , Russia and China have persistently disagreed with tough action against the regime of President Bashar al-Assad that is favored by the United States , Britain and France . The utter failure of world diplomacy and Syrian leaders to end the fighting has resulted in a humanitarian catastrophe . Opposition groups say that more than 20,000 people have died , thousands have been imprisoned by the regime and tens of thousands have been displaced from their homes . U.S. boost humanitarian aid to Syria . The conflict is now internationally regarded as a civil war between an armed resistance movement and al-Assad 's regime . `` Without serious , purposeful and united international pressure , including from the powers of the region , it is impossible for me , or anyone , to compel the Syrian government in the first place , and also the opposition , to take the steps necessary to begin a political process . You have to understand : As an envoy , I ca n't want peace more than the protagonists , more than the Security Council or the international community , for that matter , '' Annan said . The appointment of the former secretary-general in February was supported by world powers and the Syrian government . After he took the post , he helped formulate a six-point peace plan for Syria that included a cease-fire . The United Nations established the U.N . Supervision Mission in Syria to monitor the efforts to forge the plan . The plan called for establishing a cease-fire between al-Assad 's government and the opposition , allowing humanitarian groups access to the population , releasing detainees and starting a political dialogue . It also called for government forces to withdraw from city centers . Despite an initial and small drop in violence after his plan was announced , fighting raged nonstop . The terrain became so dangerous in Syria that U.N. monitors had to suspend their observation missions . In an opinion piece published Thursday in the Financial Times , Annan said regional and international players have `` clear common interests '' in a `` managed political transition . '' He said al-Assad `` must leave office . '' Read more : From rocks to RPGs . Russia , China and Iran , friends of the Syrian regime , `` must take concerted efforts to persuade Syria 's leadership to change course and embrace a political transition , realizing the current government has lost all legitimacy , '' he said . The United States , Britain , France , Turkey Saudi Arabia and Qatar must press `` the opposition to embrace a fully inclusive political process . '' `` None of this is possible , however , without genuine compromise on all sides , '' he wrote . `` Syria can still be saved from the worst calamity . But this requires courage and leadership , most of all from the permanent members of the Security Council , including from Presidents Putin and Obama . Is ours an international community that will act in defense of the most vulnerable of our world , and make the necessary sacrifices to help ? The coming weeks in Syria will tell . '' The Annan initiative disappointed opposition groups . Rafif Jouejati , English-language spokeswoman for the opposition Local Coordination Committees of Syria , said Annan 's replacement `` will be equally constrained by Assad 's failure to adhere to any sort of plan , and the ongoing complicity of Russia , China and Iran in mass killings and exodus in Syria . '' `` The Annan resignation was expected , since his plans have failed . While this may cause discomfort for the U.N. and prove that it was unable to provide a solution for the Syrian people , it means nothing to the Syrian people . The revolution will continue until we achieve our goals of freedom , democracy and dignity . `` It is sad to watch the diplomatic process die in such indignity , but millions of Syrians had indicated that Assad had no intention of implementing any diplomatic or political solution to the current crisis in Syria , '' she said . `` Annan 's plan was fatally flawed for several reasons , chief among them that 300 unarmed observers under the near-constant supervision of regime monitors could not possibly have been effective in observing , enforcing or even reporting on a cease-fire , let alone the other tenets of the Annan Plan . '' Read more : Syrian regime failed to protect civilians . Vitaly Churkin , Russian ambassador to the United Nations , said he hopes Annan uses the rest of the time on the job to be as effective as possible `` under these very difficult circumstances . '' `` We understand that it is his decision . We regret that he chose to do so . We have supported very strongly Kofi Annan 's efforts , '' he said . `` I 'm very encouraged that the secretary-general is already working on finding a successor for Kofi Annan who would be able to pick up this daunting task of putting an end to the crisis in Syria . '' Ambassador Susan Rice , the United States ' permanent representative to the United Nations , said Annan took on a `` thankless and difficult task at great personal cost . '' `` When the Security Council failed to heed Mr. Annan 's repeated calls for collective and significant consequences for non-compliance with its prior resolutions , those members who blocked this action effectively made Mr. Annan 's mission impossible , '' she said in a written statement . Catherine Ashton , the European Union 's foreign policy chief , said Annan 's resignation `` makes clear how unfortunate it has been that the U.N. Security Council was unable to agree to a resolution . '' `` The EU continues to support the efforts of the U.N. and the League of Arab States and calls for the early appointment of a successor to carry on Mr. Annan 's work towards a peaceful political transition in Syria , '' Ashton said in a written statement . Ban , who was first to announce the move , praised Annan 's `` determined and courageous efforts '' and said the news left him with `` deep regret . '' He said that Annan worked within the mandate provided to him by the U.N. General Assembly and that he is `` indebted to him and his team for all they have tried to achieve . '' But Ban also said the government and the opposition have been determined `` to rely on ever-increasing violence , '' and there have been `` persistent divisions within the ( U.N. ) Security Council . '' `` Kofi Annan deserves our profound admiration for the selfless way in which he has put his formidable skills and prestige to this most difficult and potentially thankless of assignments , '' Ban said . Annan had been secretary-general from 1996 to 2006 and was involved in other initiatives , including the mediation of the election dispute a few years ago in Kenya . Ban said peace can come only if there 's a `` firm commitment to dialogue '' among the players in Syria and international unity . `` Tragically , the spiral of violence in Syria is continuing . The hand extended to turn away from violence in favor of dialogue and diplomacy -- as spelled out in the six-point plan -- has not been taken , even though it still remains the best hope for the people of Syria . '' Complete coverage of the stories unfolding in Syria . The CNN Library contributed to this report .
NEW : The EU ` ` calls for the early appointment of a successor '' for Algeria . U.N. Secretary - General Ban Ki - moon announced the move with ` ` deep regret '' . League Secretary - General Ahmed El Araby and others are calling for a successor . U.S . Ambassador : Security Council vetoes made Annan 's mission impossible .