Gibraltar’s chief minister, Fabian Picardo, has flown to London to hold talks with Theresa May about the possible release of an Iranian-owned tanker that was seized a fortnight ago by the Royal Marines.

The detention of the Grace 1 and its cargo of 2.1m barrels of oil, which was believed to have been headed to Syria in breach of EU sanctions, has turned into a focal point of severe friction between Tehran and London as Iran threatens to take further steps way from the nuclear deal it signed in 2015 with the US, China, Russia and 3 EU states.

The UK has been under pressure from some European and Gulf allies to release the ship. Suggested by the US, the capture occurred on 4 July when the ship unexpectedly put down an anchor in what the UK regards as its waters. Spain monitored the capture.

Iranian sources say its intermediaries took up a weekend offer from Picardo to meet to discuss the situation.

Speaking in the Commons, the UK’s Middle East minister, Andrew Murrison, said decisions about the Panamanian-flagged tanker were a matter for Picardo. But in reality the Foreign Office will have influence over the key strategic decisions around whether the retention of the ship is either legally or diplomatically wise.

The foreign secretary, Jeremy Hunt, following consultations with Picardo and the Iranian foreign minister, Javad Zarif, offered on Saturday to release the ship as long as Iran gave assurances that the ship’s oil was not bound for Syria, or any other entity under EU sanctions.

In a statement, Picardo said: “We would be prepared to facilitate the release of the detained assets to the rightful owner if we received guarantees that it would not be going to Syria.”

Speaking to the BBC’s Hardtalk programme in New York on Monday, Zarif said the ship had never been bound for Syria, but would not identify the true destination.

He said: “We announced from the very beginning this ship was not going to Syria.” Asked where it was going, he said: “It was going to a place in the Mediterranean other than Syria. We made it clear.”

Iran has been exporting oil to western Turkey, and looked to have resumed oil sales to Syria after a gap of more than a year in March this year.

Zarif said he would “do anything to avoid the US knowing what we are doing”, pointing out that the US was unilaterally attempting to force Iran back to the negotiating table by trying to impose an unprecedented ban all Iranian oil exports.

“The UK by confiscating our ship is helping the US in imposing its illegal oil sanctions against Iran,” said Zarif.

It is not clear if Iran has been able to give stronger assurances through other private diplomatic channels that would satisfy the Gibraltar supreme court about the oil’s destination.

Iran believes it is not party to the EU sanctions on Syria, one of its close allies, and has described the ship’s seizure as unlawful and an act of piracy.

Some independent observers say it is unusual for the UK to have intervened to force a non-EU state to comply with EU sanctions.

Gibraltar’s authorities arrested the Indian captain and first officer of the Grace 1, but have not pressed any charges.