Germany and France on Saturday denounced Iran's seizure of a British-flagged tanker in the Strait of Hormuz and called on the country to release the vessel immediately.

"This is an unjustifiable interference in commercial shipping that dangerously escalates an already tense situation," the German Foreign Office wrote on Twitter.

France's Foreign Ministry said it "strongly condemned" the seizure and expressed its "full solidarity with the United Kingdom," according to the Reuters news agency.

Iran's state news agency reported that authorities had detained the Stena Impero and its 23 crew on Friday after it allegedly collided with a local fishing boat.

Read more: Opinion: Iran's calculated escalation in the Persian Gulf

Armed Iranian guards also briefly boarded a Liberian-flagged tanker before they allowed the vessel to continue.

The British Foreign Office summoned the Iranian Charge d'Affaires on Saturday to discuss the seizure. Foreign Secretary Jeremy Hunt said the move "shows worrying signs Iran may be choosing a dangerous path of illegal and destabilizing behavior."

The seizure came hours after a court in Gibraltar extended the detention of Iranian tanker Grace 1, which the British territory captured in early June over fears that it was illegally transporting oil to Syria.

Iran threatened to seize British tankers in retaliation, prompting the UK to send an additional warship to the Persian Gulf.

Tensions in the Gulf are already running high following the US's claim that it shot down an Iranian drone, fallout over the US withdrawal from a 2015 nuclear deal with Iran and US accusations that Iran attacked several tankers earlier this year.

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