President Hassan Rouhani on Monday departed Tehran for New York to attend the United Nations General Assembly on a mission to win Iran support against "cruel" pressure from arch-foe the United States.

Speaking to reporters before boarding his flight, Rouhani said his delegation was heading to the UN gathering despite reluctance from US President Donald Trump's administration to issue them visas.

Tehran and Washington have been at loggerheads since May last year when Trump abandoned a 2015 nuclear deal and began reimposing sanctions on Iran in a stated campaign of "maximum pressure".

"When the Americans aren't willing [to let Iran participate], we must insist on travelling," Rouhani said.

"It is essential for us to take part in the UN General Assembly and talk at various levels," he told a news conference at Tehran's Mehrabad airport.

"The cruel actions that have been taken against the Iranian nation and also the difficult and complicated issues that our region faces with them need to be explained to the people and countries of the world."

Rouhani said he would present an Iranian plan for peace in the Gulf to the UN General Assembly.

'Hormuz Peace Endeavour'

Under the plan dubbed the Hormuz Peace Endeavour, or HOPE, he said: "All the coastal states of the Persian Gulf are invited to join this coalition to provide and maintain regional security."

Tensions have flared in the Gulf since May when Iran began reducing its commitments under the nuclear deal and the US deployed military assets to the region.

Tensions have escalated further in the wake of devastating September 14 attacks on Saudi oil installations that Washington and Riyadh have, to varying degrees, blamed on Tehran.

British Prime Minister Boris Johnson said "the UK is attributing responsibility with a very high degree of probability to Iran" for the attacks on Saudi oil infrastructure, the UK's Press Association news agency reported on Monday.

Johnson was himself en-route to New York, where he was scheduled to meet with Iran's president.

Since pulling out of the nuclear deal, the US has slapped waves of sanctions on Iran that have targeted its armed forces, financial sector and senior officials, including Foreign Minister Mohammad Javad Zarif.

Iran has responded by scaling back its commitments under the 2015 deal that gave it the promise of sanctions relief in return for limiting the scope of its nuclear programme.

The US has said it would make its case against Iran at the UN meeting.

Secretary of State Mike Pompeo said the US wanted to give diplomacy "every opportunity to succeed" in the wake of the attacks that set aflame Saudi Arabia's Abqaiq plant and the Khurais oil field.

"Our administration's taking this on in a serious way and we are working diligently to see that this has a diplomatic outcome," he told ABC's "This Week" on Sunday.

"But make no mistake about it, if we're unsuccessful in that and Iran continues to strike out in this way, I am confident that President Trump will make the decisions necessary to achieve our objectives."