Iranian President Hassan Rouhani set off for New York on Monday to attend the UN General Assembly on a mission to win Iran support against "­cruel" pressure from arch-foe the US.

His departure came as Iran said an oil tanker flying the flag of US ally Britain was "free" to leave more than two months after its forces seized it in sensitive Gulf waters.

Speaking 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 subsequently 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 traveling," 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."

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

The US has since formed a naval coalition with its allies Britain, Australia, Bahrain, Saudi Arabia and the United Arab Emirates to escort commercial ships, in response to a spate of incidents in the Gulf.

The tensions 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 Monday his government was drawing a similar conclusion to its allies.

"The UK is attributing responsibility with a very high degree of probability to Iran" for the Saudi attacks, he said, quoted by the UK's Press Association news agency.