ISLAMABAD (Reuters) - Pakistani Prime Minister Yusuf Raza Gilani appointed a new director-general of the country's powerful spy agency on Friday, Gilani's office said in a statement, possibly allowing a fresh approach to U.S.-Pakistan relations after 15 months of tension. Lieutenant-General Zaheer-ul-Islam will take over as director-general of the Directorate of Inter-Services Intelligence (ISI) after serving as the commander of the V Corps, one of the most important in the army and based in Karachi, Pakistan's largest city and its commercial hub. "It is very important at this time when the Americans and NATO troops are withdrawing from Afghanistan, it is important to have a professional man running the ISI," military analyst Talat Masood said. Islam takes over from Lieutenant-General Ahmad Shuja Pasha, who was appointed in 2008 and oversaw some of the stormiest times in the U.S.-Pakistan relationship. He is scheduled to retire on March 18. Analysts say the incoming director-general has held some of the most important posts in the army since his commissioning in 1977, and is seen both inside and outside the military as experienced and professional. " (Islam) has the confidence of the people who matter," Masood said. The institution also needed to change; this injects new thinking and energy into the organization. Others, however, believe policy within one of Pakistan's most powerful institutions will remain unchanged and will still be directed by the powerful chief of army staff, General Ashfaq Kayani. The role of the ISI does not necessarily depend on an individual, but it's a policy that is designed primarily by the army chief. I think there will be continuity of policy," said political analyst Hasan Askari Rizvi. Either way, the appointment may be an opportunity to set a new tone in the often tense dialogue between Pakistan and the United States. "The new person can make a new beginning because he doesn't have the baggage of problems with the U.S.," Rizvi said. So in a way, this can facilitate the improvement of the relationship. The head of the ISI occupies one of the most important posts in Pakistan, at the intersection of domestic politics, the war on militancy and Pakistan's foreign relations. Pakistan is crucial to U.S. efforts to stabilize neighboring Afghanistan. If, as alleged, the ISI has ties to the Afghan Taliban and other insurgent groups fighting U.S.-led NATO troops in Afghanistan, it may be able to bring them to the negotiating table - though Pakistan denies having ties to militant groups. Without Pakistan's help, Afghanistan could descend into civil war again after foreign troops withdraw at the end of 2014. Pasha's departure is likely to be a relief to the American intelligence community which had a frosty relationship with him, one that became even more difficult after U.S. special forces found and killed al Qaeda leader Osama bin Laden in a town about a two-hour drive from the ISI's Islamabad headquarters in May last year. The tensions between the American and Pakistani spy agencies in recent years were a marked reversal from the 1980s, when the CIA and the ISI cooperated to roll back the Soviet invasion of Afghanistan. Gilani was likely happy to appoint someone other than Pasha, who was rumored to be up for a third extension to his tenure. Pakistan was thrown into turmoil on October 10 last year when businessman Mansoor Ijaz wrote in the Financial Times that a senior Pakistani diplomat had asked that a memo be delivered to the Pentagon with a plea for U.S. help to stave off a feared military coup following the bin Laden raid. Ijaz claimed in his BlackBerry messages published in a local Pakistani newspaper that after the bin Laden raid, Pasha had secured support from some unspecified Gulf governments to remove President Asif Ali Zardari. When the British daily The Independent wrote a story on the allegation, the ISI strongly denied it. "He (Pasha) has become very controversial in the last couple of months," retired brigadier Shaukat Qadir, a friend of Pasha and twice his instructor at the National Defence University, said dryly. Additional reporting by Qasim Nauman in Islamabad; Writing by Rebecca Conway; editing by Tim Pearce