US-Iranian tensions have increased since Trump's decision last year to abandon the 2015 nuclear deal

Iran on Thursday signalled a willingness to engage in diplomacy to defuse tensions with the United States with a modest offer on its nuclear programme that met immediate scepticism in Washington.

Iran's foreign minister told reporters in New York that Iran could immediately ratify a document prescribing more intrusive inspections of its nuclear programme if the United States abandoned its economic sanctions, media organizations reported.

The document, known as the Additional Protocol, gives U.N. inspectors from the International Atomic Energy Agency (IAEA) more tools to verify that a nuclear programme is peaceful.

While U.S. officials suggested they viewed the idea as a non-starter, analysts said it could provide an opening for U.S. President Donald Trump's administration to pursue diplomacy.

"If Trump wants more for more, we can ratify the Additional Protocol and he can lift the sanctions he set," the Guardian newspaper quoted Iranian Foreign Minister Mohammad Javad Zarif as telling reporters.

However, since Iran is already implementing the protocol and has often offered in the past to ratify it, it was not clear that Zarif's proposal constituted much of a concession.

Under the 2015 nuclear deal agreed to by Tehran, Iran must seek ratification of the protocol eight years after the deal was adopted. That would be the same time that the United States must seek permanent termination of many of its sanctions on Iran.

U.S. officials responded sceptically, suggesting it was a disingenuous effort to get sanctions relief.

"Their whole game is to try to get any sanctions relief they can while maintaining the ability to get a nuclear weapon in the future," said an official on condition of anonymity, saying Iran was "trying to spin a small action into" something bigger.

The official noted that under the offer, Iran would keep enriching uranium, a process that can produce fissile material for nuclear weapons, and would do nothing to rein in its support for regional proxies in Yemen, Iraq, Syria and Lebanon.

A second U.S. official was also dismissive.

"If Iran wants to make a serious gesture, it should start by ending uranium enrichment immediately and having an actual decision maker attempt to negotiate a deal that includes a permanent end to Iran's malign nuclear ambitions, including its development of nuclear-capable missiles," said the official.

'A Creative Opening'?

Former U.S. officials saw a diplomatic opening.

"If the foreign minister has suggested that the Majlis (the Iranian parliament) would ratify the additional protocol now, that is a serious step," said Wendy Sherman, a former Obama administration official who negotiated the 2015 nuclear deal.

"Of course, Iran will want something serious in return. Nonetheless, a creative opening," she added.

Richard Nephew, a former U.S. official now at Columbia University, said Zarif surely knew Washington would reject his idea. But he said it signalled Iran wants a diplomatic solution and suggested Iran has no intent to throw out IAEA inspectors.

"The administration ought to use this as an opportunity to talk seriously internally about what it wants and to test the Iranian position, but I doubt that they will," he said.

U.S.-Iranian tensions have increased since Trump's decision last year to abandon the nuclear deal that Iran struck with six major powers in 2015 under which it agreed to rein in its atomic programme in return for broad relief from economic sanctions.

Relations have deteriorated since May, when Trump tightened U.S. sanctions on Iran to try to choke off its oil exports, the main source of foreign exchange and government revenues for the Islamic Republic.

Washington is trying to force Tehran to agree to stricter limits on its nuclear capacity, curb its ballistic missile programme and end support for proxy forces in a regional power struggle with U.S.-backed Gulf Arab states.

Fears of direct U.S.-Iranian conflict have risen since May with several attacks on oil tankers in the Gulf, Iran's downing of a U.S. surveillance drone and a plan for U.S. air strikes on Iran last month that Trump called off at the last minute.

Trump on Thursday said a U.S. Navy ship had "destroyed" an Iranian drone in the Strait of Hormuz after the aircraft threatened the ship. But Zarif told reporters at the United Nations he was not aware of any Iranian drone being downed.