The leader of Britain's main opposition party has questioned whether the government had evidence to back up its accusations that Iran was behind attacks on oil tankers in the entrance to the Gulf, and warned against escalating tensions.

Britain on Friday joined the United States in blaming Iran for attacks on two tankers, raising fears of a broader military confrontation in a vital passageway for the world's oil industry.

Iran has denied any involvement.

"Without credible evidence about the tanker attacks, the government's rhetoric will only increase the threat of war," Labour leader Jeremy Corbyn wrote on Twitter late on Friday.

"Britain should act to ease tensions in the Gulf, not fuel a military escalation that began with US withdrawal from the Iran nuclear agreement," he added," he said, referring to Washington's withdrawal from a 2015 pact to curb Tehran's nuclear plans.

Foreign Minister Jeremy Hunt, who is one of the leading candidates to succeed British Prime Minister Theresa May after she announced she would step down, described Corbyn's comments as "pathetic and predictable."

"Why can he never bring himself to back British allies, British intelligence or British interests?," Hunt said.

On Friday, the United States released video footage they say shows Iranian special forces removing an unexploded mine from one of the tankers.

Dominic Raab, another candidate to be the next Conservative leader, said Corbyn's comments show he is unfit to lead Britain.

"Corbyn allows his anti-American prejudice to skew his moral compass and political judgment," he said.

Corbyn last year was criticized by opponents and lawmakers in his own party after he questioned the government's decision to blame Russia for a nerve toxin attack on a former double agent in England.

The shadow foreign secretary, Emily Thornberry, said on Saturday that independent evidence should be brought forward that shows who was responsible for the attacks but warned the government from getting dragged into a large-scale war.

Speaking to BBC Radio 4's Today, Thornberry said "these are extremely dangerous developments and we really have to pause and think about where we are going next" and that there are times when the government should not follow the U.S.

"We made a dreadful mistake when it came to Iraq and we must not make the same mistake again," she said.

On Friday, German Foreign Minister Heiko Maas said that a video provided by the U.S. was not sufficient to prove that Iran was behind the tanker attacks.

U.S. President Donald Trump has blamed Iran for the attacks, citing a U.S. Central Command video that purports to show a patrol boat with personnel docked near one of the oil tankers.

Dozens of crew members on Friday were rescued after the Japanese Kokuka Courageous and Norwegian Front Altair oil tankers burst into flames as they were passing through the Strait of Hormuz and entering the Gulf of Oman.