President Hassan Rouhani was headed for New York on Monday to attend the UN General Assembly on a mission to win support for Iran against "cruel" pressure from its arch-foe the United States.
His departure came as Tehran said an oil tanker flying the flag of US ally Britain was "free" to leave more than two months after seizing it in sensitive Gulf waters.
Before departing, Rouhani said his delegation was heading to the UN gathering despite Washington's reluctance to issue them US visas.
Tehran and Washington have been at loggerheads since May last year when President Donald Trump abandoned a 2015 nuclear deal and subsequently began reimposing sanctions on Iran in a stated campaign of "maximum pressure".
"It is essential for us to take part in the UN General Assembly and talk at various levels," Rouhani said 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."
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 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 after devastating September 14 attacks on Saudi oil installations that Washington and Riyadh have, to varying degrees, blamed on Tehran.
Britain's Prime Minister Boris Johnson said Monday that his government was drawing a similar conclusion.
"The UK is attributing responsibility with a very high degree of probability to Iran" for the Saudi attacks, he said, quoted by Britain's PA news agency.
Johnson was himself en route to New York, where he was scheduled to meet with Rouhani.
Iran slammed Britain over the accusation.
"Instead of fruitless efforts against the Islamic Republic of Iran, Britain must stop selling deadly weapons to Saudi Arabia... and free itself from accusations of committing war crimes against Yemenis," foreign ministry spokesman Abbas Mousavi said, quoted by Tasnim news agency.
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's government spokesman Ali Rabiei said Monday the tanker was "free" to leave after the completion of legal proceedings, without specifying when it might set sail.
The announcement comes after a court in British overseas territory Gibraltar ordered the release of an Iranian oil tanker in mid-August.
Before flying to New York, Rouhani said Iran would put forward a Gulf peace plan at 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".
'Something can happen'
Since pulling out of the nuclear deal, Washington has slapped waves of sanctions on Iran, including against its supreme leader Ayatollah Ali Khamenei and 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 UN.
Secretary of State Mike Pompeo said the US wanted to give diplomacy "every opportunity to succeed" in the wake of the Saudi attacks.
After the attacks, the US announced further sanctions on Iran that dealt a blow to French efforts to arrange a Rouhani-Trump meeting.
Iran on Monday reiterated its conditions for any talks with the US.
"If the US is ready to end sanctions and come back to the conditions of the nuclear agreement, the way would be open for us to make a decision," said Rabiei.
"One of these decisions could be negotiations," the spokesman said.
French President Emmanuel Macron held out hope of a breakthrough despite the Saudi attacks.
"Did the chances of a meeting increase with these strikes? No, we have to be clear, we can see that things are getting tense," he said.
But "the two main players are there", he said, referring to the presence of Rouhani and Trump in New York. "Something can happen."