UK Prime Minister Phones Iran's President Prime Minister David Cameron contacted Iranian President Hassan Rouhani on Tuesday ahead of negotiations over Tehran's nuclear program in what Downing Street says was the first such conversation between the leaders of the two countries in more than a decade. The telephone call comes a day before Britain, the U.S. and four other world powers resume talks with Iran in Geneva - and follows a concession by Tehran dropping its insistence that the six world powers publicly acknowledge its claim to have the right to enrich uranium. That move opens a way to focus on more practical steps on the road reaching an initial agreement that freezes Iran's nuclear program in exchange for sanctions relief. Cameron's office said Tuesday that the prime minister and Rouhani agreed during their telephone conversation Tuesday that significant progress had been made in recent Geneva negotiations over the nuclear issue and that it is important to "seize the opportunity" in the next round of talks. "The prime minister underlined the necessity of Iran comprehensively addressing the concerns of the international community about their nuclear program, including the need for greater transparency," Downing Street added in a statement. Iran insists that it wants to enrich uranium for energy and medical purposes. But the U.S. and its allies - Britain included - contend that Tehran's nuclear program has a military dimension. British-Iranian relations have been hampered by arguments over the Islamic Republic's nuclear program and Tehran's support for embattled leader Bashar Assad in Syria. But in a sign of thawing relations, the U.K. and Iran last week appointed non-resident charge d'affaires offices to work toward the eventual reopening of the British embassy in Iran and the Iranian one in Britain. The British embassy in Tehran was closed in late 2011 after a mob overran the building as tensions over a possible attack on Iran's nuclear facilities ran high. Follow Cassandra Vinograd at http://twitter.com/CassVinograd