By Holly Yan and Saad Abedine, CNN July 15, 2012 -- Updated 0900 GMT (1700 HKT) NEW: At least 14 people are killed across Syria on Sunday, opposition activists say NEW: The Syrian regime says two "terrorists" confessed to killings in Tremseh Abdulbaset Sieda calls for the U.N. Security Council to force the regime to stop killing U.N. monitors will return to the town of Tremseh, the site of a reported massacre, on Sunday (CNN) -- The head of a prominent Syrian opposition group says U.S. President Barack Obama should take greater action on Syria and not be worried about whether the "right decision" will hurt Obama's re-election campaign. The plea comes after more than 200 people were reportedly massacred last week in the Syrian town of Tremseh. We want for America and the Western countries to carry out their responsibilities through the (U.N.) Security Council and work to adopt a resolution under Article 7 to force this regime to stop killing Syrians," Abdulbaset Sieda, chairman of the Syrian National Council, said in an interview to air Sunday on CNN's Fareed Zakaria GPS. With regard to America, specifically, we would like to say to President Obama that waiting for election day to make the right decision on Syria is unacceptable for the Syrians. We cannot understand that a superpower ignores the killing of tens of thousands of Syrian civilians because of an election campaign that a president may win or lose. That's why we are saying there is work that must take place at the Security Council. U.S. officials have decried the Syrian regime and called for President Bashar al-Assad to step down. But previous attempts to pass resolutions condemning al-Assad's regime at the U.N. Security Council have failed due to vetoes from Russia and China. If continued efforts at the Security Council fail, the United States and other countries "can move outside the scope of the Security Council," Sieda said. While world leaders remain without a consensus about what to do in Syria, the death toll continued to escalate, opposition activists said. At least 14 people were killed Sunday in cities across the country, the opposition Local Coordination Committees of Syria said. The deaths include five people killed by shelling in Hama and Deir Ezzor, the LCC ssaid. On Sunday, U.N. observers will return to the town of Tremseh, where opposition activists say more than 200 people were killed Thursday -- the deadliest day in Syria's 16-month crisis. "The attack ... appeared targeted at specific groups and houses, mainly of army defectors and activists," said Sausan Ghosheh, spokeswoman for the head of the U.N. monitoring mission. There were pools of blood and blood spatters in rooms of several homes together with bullet cases. Meanwhile, the state-run Syrian Arab News Agency reported that two terrorists "confessed to taking part in a series of murders and acts of terrorism before the army entered the town" of Tremseh. The Syrian regime has blamed "armed terrorist groups" for the massacre in Tremseh, saying residents called security forces for help after the terrorist groups raided the town. But the bloodbath prompted renewed international outrage against the Syrian regime and underscored the desperate need to find a solution to the conflict that has killed thousands. Since the Syrian conflict began in March 2011, the United Nations estimates more than 10,000 people have been killed in the violence; opposition groups say thousands more have died. Sieda said it's important to remember that despite surging casualty tolls, each victim's death is a tragedy. We are dealing with numbers and forget that these people are human beings who have fathers, mothers, brothers, sisters and friends.