US Secretary of State Condoleezza Rice has met Tony Blair at Chequers for private talks over Iran and Iraq.
Condoleezza Rice predicted the insurgency in Iraq would fail
"They discussed their shared concern about the need for Iran to meet its international obligations," a Downing Street spokesman said.
Iraq's constitution referendum, in which millions voted on Saturday, and security situation were also discussed.
"They were greatly encouraged by the level of participation in the referendum," the spokesman said.
Both leaders were looking forward to "the political process continuing through to elections in December, and beyond", he added.
Ms Rice's visit was part of a European tour to gather support for encouraging Iran to return to international talks over its nuclear programme.
During her visit Ms Rice told the BBC Iran should end support of the insurgency in Iraq, and become "good neighbours" instead.
The Iranian ambassador in London, Dr Seyed Mohammed Hossein Adeli, insisted his country did not support the use of violence against British troops in Iraq.
He also denied his country was causing trouble in Iraq as a response to criticism of its nuclear programme.
He told the BBC Britain and Iran had talked about the attacks through diplomatic channels, and he was suspicious that Britain was highlighting its complaints to put pressure on Iran.
Ms Rice met Mr Blair at his country residence, Chequers
He rejected Washington's belief that Iran wanted to develop nuclear weapons, and said it needed nuclear energy to replace oil stocks when they ran out.
UK Foreign Secretary Jack Straw, speaking separately to the BBC, insisted Britain had evidence linking Iran or Hezbollah to insurgent activity in Iraq.
"What we have presented to the Iranians is evidence, which in our judgement clearly links the improvised explosive devices which have been used against British and other troops mainly in the south of Iraq, to Hezbollah and Iran.
"We look to the Iranians to desist from anything they have been involved with in the past and to use their very considerable influence with Hezbollah to ensure this continued use... stops in Iraq."
Mr Straw also said the British and American governments were in agreement over how to deal with Iran's nuclear programme.
He said: "My own belief is that military action in respect of the nuclear dossier is inconceivable.
"One of the reasons that it is inconceivable is that it is not on anybody's agenda."
London was the final stop of Ms Rice's tour, and she left for the US on Sunday afternoon.
Her visit came as the British embassy in Tehran issued a statement strongly denying involvement in the bombing of a shopping centre in Ahwaz on Saturday.
Four people died and many more were injured in the city, near Iran's south-western border with Iraq, Iranian state television reported.
Iranian officials said they suspected British involvement, but the British embassy condemned the blasts and rejected the idea that Britain was in any way involved.