Rouhani heads to UN in bid to win support against US

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 United States.

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 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 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."

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.

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

Long-fraught relations between London and Tehran were further soured after Iran seized the British-flagged Stena Impero on July 19 for allegedly breaking "international maritime laws".

Iran said on Monday that the tanker was "free" to leave after the completion of legal proceedings.

But government spokesman Ali Rabiei did not specify when the vessel, which is owned by a Swedish company, would set sail.

The Iranian announcement comes after a court in British overseas territory Gibraltar ordered the release of an Iranian oil tanker in mid-August despite an 11th-hour US legal bid to keep it in detention.

Before departing for New York, Rouhani said Iran would put forward a plan to de-escalate tensions in the Gulf during the UN meeting.

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".

Rouhani said the Americans were "at the root" of conflicts in the region and their motive for blaming Iran was to deploy its forces in the Gulf and have access to its oil.

"It is clear that they want to own all of the oil that is in the east of Saudi Arabia.

"It is clear that the US has other goals and such incidents are their pretext to be more present in the region."

Since pulling out of the nuclear deal, Washington has slapped sanctions on Tehran's armed forces, financial sector and top officials including Foreign Minister Mohammad Javad Zarif.

Iran has responded by scaling back its commitments under the 2015 deal with world powers 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 General Assembly.

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 Abqaiq plant and the Khurais oil field in eastern Saudi Arabia.

Following the attacks, the US announced it was imposing further sanctions on Iran's central bank.

That may have dealt a final blow to efforts by France to arrange a meeting between Rouhani and Trump during the UN General Assembly.