The Iranian tanker caught in a stand-off between Tehran and the West has left Gibraltar, according to shipping data, hours after the British territory rejected a US request to detain the vessel further.

British Royal Marines seized the tanker in Gibraltar in July on suspicion that it was carrying oil to Syria, in violation of European Union sanctions.

That triggered a series of events that have heightened tensions on international oil shipping routes through the Gulf.

The Grace 1, reflagged as the Adrian Darya 1, left anchorage off Gibraltar about 11pm on Sunday (local time).

It sailed despite reportedly needing repairs that might have impeded its departure, according to a lawyer for the ship’s captain.

The tanker’s detention ended last week, but on Friday a federal court in Washington issued a warrant for the seizure of the ship, the oil it carries and nearly $1 million.

#BREAKING The #Iranian tanker Grace-1 has begun sailing and is now leaving #Gibraltar: Iranian State TV pic.twitter.com/9tWtuMaKry

— EHA News (@eha_news) August 18, 2019

Gibraltar said on Sunday it could not comply with that request because it was bound by EU law.

“The EU sanctions regime against Iran – which is applicable in Gibraltar – is much narrower than that applicable in the US,” the government said in a statement.

Washington had attempted to detain the Grace 1 on the grounds that it had links to Iran’s Islamic Revolutionary Guard Corps, which it has designated a terrorist organisation.

Iran has denied the tanker was ever headed to Syria.

-with AAP