British Prime Minister David Cameron telephoned the Iranian president Sheikh Hassan Rouhani on Tuesday, becoming the first British premier to call an Iranian leader in over a decade, Downing Street said.

British Prime Minister David Cameron telephoned the Iranian president Sheikh Hassan Rouhani on Tuesday, becoming the first British premier to call an Iranian leader in over a decade, Downing Street said.

Cameron and Rouhani discussed the upcoming negotiations over Iran's nuclear program, the conflict in Syria and the recent improvements in bilateral ties between London and Tehran, it said in a statement.

"The prime minister became the first British prime minister to call the Iranian president in more than a decade today when he spoke to President Rouhani this afternoon, ahead of this week's nuclear negotiations in Geneva," a Downing Street spokesman said.

They "agreed to continue efforts to improve the relationship on a step by step and reciprocal basis" following the appointment of non-resident charge d'affaires to each other's countries last week, the statement said.

The appointment of the envoys was a key step towards improving ties severed after Iranian protesters ransacked the British embassy in 2011.

"On Iran's nuclear program, both leaders agreed that significant progress had been made in the recent Geneva negotiations and that it was important to seize the opportunity presented by the further round of talks which get underway tomorrow," the statement said.

Cameron "underlined the necessity of Iran comprehensively addressing the concerns of the international community about their nuclear program, including the need for greater transparency", it said.

"On Syria, there was agreement on the need for a political solution to end the bloodshed," the Downing Street statement said.