Iran's supreme leader, Ayatollah Ali Khamenei, has said relations with the US could be restored in the future.
Ayatollah Khamenei maintained a hard line on uranium production
In a speech to students, he said the time was not right to restore ties, but if it were ever in Iran's interests he would endorse such a move.
The US and Iran cut their diplomatic ties after the 1979 Islamic revolution and the subsequent takeover of the US embassy by militants in Tehran.
Relations have been further strained by the row over Iran's nuclear programme.
"We have never said these relations should be suspended indefinitely," said Ayatollah Khamenei.
But he said now was not the right time because it would be harmful to Iran.
Restored diplomatic ties would provide "an opportunity for US infiltration, traffic of their intelligence agents and espionage of Iran".
Last month, US Secretary of State Condoleezza Rice said Washington was open to better relations with Tehran if it stopped enriching uranium.
Ayatollah Khamenei maintained his country's uncompromising line on the nuclear programme, saying his country would not halt enriching uranium.
Iran says its nuclear programme is for peaceful electricity generation only, but the US and several European allies fear Tehran is trying to build nuclear weapons.
The Ayatollah's words are unlikely to appear conciliatory in Washington and London says the BBC's Middle East analyst Roger Hardy.
Rather, they were probably directed at a domestic audience ahead of parliamentary elections in March.
The Ayatollah was also underlining the fact that he, and he alone, has the final word on foreign policy, says our analyst.