Britain is seeking to establish whether Iran has sold oil to Syria in breach of written undertakings given by Tehran to authorities in Gibraltar.

Iran’s foreign ministry said on Sunday that a tanker seized by British Marines on 9 July and released in August had reached its final destination “on the Mediterranean coast” and sold its oil – without identifying the country.

However, Adrian Darya, previously called Grace 1, has been seen off the coast of Syria for the past three days.

The supertanker was detained off Gibraltar for allegedly breaking EU sanctions on Syria. On 15 August a court in Gibraltar released it when Iran gave assurances it would not sail to Syria, despite pressure from the US for the ship’s continued detainment.

The US Department of Justice issued a warrant for the seizure of the ship, and the US Department of State has confirmed that US officials subsequently offered the ship’s Indian captain millions to take the ship’s oil to a port where the oil – worth as much as as £110m – could be impounded by the US. After the captain gave no positive response, he and the ship were put under US sanctions.

Since leaving Gibraltar the ship had taken a peripatetic route towards Syria, but was last photographed off the Russian navy port of Tartus in Syria. TankerTrackers, a firm that monitors oil tankers, has seen no evidence that its 2.1m barrels of oil have been discharged.

“We will continue to put pressure on Iran and as President Trump said, there will be no waivers of any kind for Iran’s oil,” US Treasury official Sigal Mandelker told Reuters on Sunday.

The UK argued that an EU embargo on oil sales to Syria entitled the UK to impound the tanker once it entered British waters. Iran says it is not subject to the EU sanctions regime.

In apparent retaliation for the seizure of the tanker, the Iranians seized the British-flagged vessel Stena Impero in the strait of Hormuz. On Sunday the Iranian foreign ministry said that ship would be released soon, adding that preparations for the ship’s release were at their last stages. Last week it released seven of the ship’s crew. Sixteen of the crew, none of whom were British, remained on board, and Stena Bulk, the owner of Stena Impero, said last week it was working tirelessly to secure their release, adding there was no evidence the ship had breached any maritime regulations.

It is not clear if the UK would have any powers if indeed the Iranian oil has been sold to Syria, but the episode will hardly do much to rebuild shattered trust between Tehran and London, and will be used by Washington hawks to argue Iran’s word cannot be taken at face value.

John Bolton, the US national security advisor, tweeted last week that anyone who thought the oil was not bound for Syria was in denial.

Privately, some British diplomats admit it was a mistake to succumb to US pressure to capture Grace 1, given the high stakes wider negotiations between Europe and Iran aimed at trying to keep Iran inside the 2015 nuclear deal.

Tehran on Saturday detailed its third planned set of steps to reduce its obligations under the deal, saying it could not stay inside the deal unilaterally. It said it had begun injecting uranium gas into advanced centrifuges and that the country will no longer abide by the deal’s limits on its nuclear research and developments.

A French plan for a $15bn credit line to help the Iranians sell their oil appears to have foundered because of US objections to reintroducing oil export waivers, a precondition for the plan gaining traction. The credit line was seen as a way to start talks between Washington and Tehran on a reworked nuclear deal.