(CNN) - An Iranian oil supertanker seized by the UK in Gibraltar last month amid escalating tension between Tehran and the West was released Sunday, according to local media and a maritime tracking site.
The Adrian Darya 1, formerly known as the Grace 1, set sail under an Iranian flag for an unknown destination.
The move came after the United States issued a warrant for the ship's seizure on Friday, in what appeared to be a last-ditch attempt to stop the tanker from moving on.
Gibraltar said on Sunday it was unable to comply with the request to continue detaining the vessel and its cargo due to differences in sanction laws between the European Union and the US.
Iran's ambassador to Britain, Hamid Baeidinejad, wrote on Twitter that the tanker was expected to depart on Sunday night, adding that two engineering teams had been flown to Gibraltar to assist.
The release of the ship comes more than a month after British Royal Marines and Gibraltar authorities stormed the tanker because it was believed to have been carrying oil to Syria, which would be in breach of EU sanctions.
It was seized July 4 in territorial waters of Gibraltar, a British overseas territory on the edge of southern Spain.
Iran denied the ship was headed for Syria. Two weeks after the tanker was taken, Iran seized a British-flagged tanker, the Stena Impero, in the Strait of Hormuz, in what has been widely regarded as an act of retaliation.
Tensions between Iran, the UK and US ratcheted up.
The Supreme Court in the British territory of Gibraltar last week approved the release of the Grace 1, which was seized off the country's coast by authorities last month, after officials said they no longer wished to detain it.
Gibraltar said it had received assurances from Iran and the owners of the oil that, were the tanker to be released, its cargo would not be taken to Syria, which would be in breach of European Union sanctions.
The Adrian Darya's owner said the released Iranian tanker would head to the Mediterranean, Iran's semi-official Mehr News Agency, citing Iranian Maritime official Jalil Eslami, reported last week.
The US also got involved in the standoff when the Justice Department applied to extend the Iranian tanker's seizure and block its release.
The State Department said in a statement late Thursday that it had deemed the ship to be "assisting" Iran's Islamic Revolutionary Guard Corps (IRGC), which the US has designated a Foreign Terrorist Organization, in evading sanctions.
But Gibraltar declined to comply, saying that Europe and the UK did not share that designation for the IRGC.
"The EU sanctions regime against Iran -- which is applicable in Gibraltar -- is much narrower than that applicable in the US," the Government of Gibraltar said in a statement Sunday.
CNN's Karen Smith, Tim Lister and Gena Somra contributed to this report.
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