British Prime Minister Keir Starmer said on Monday that whatever the pressure, Britain would not be dragged into the Iran war nor be involved in a blockade of the Strait of Hormuz.

"We're not supporting the blockade," he told BBC Radio 5 Live, adding that it was vital to get the Strait reopened.

"It is, in my view, vital that we get the Strait open and fully open, and that's where we've put all of our efforts in the last few and we'll continue to do so," Starmer said.

Britain had minesweepers in the region, he said, and while he could not discuss operational matters, the military capability was "focused from our point of view on getting the Strait full open".

The U.S. military said it would begin a blockade of all maritime traffic entering and exiting Iranian ports and coastal areas on Monday, after weekend talks failed to reach a deal to end the war with Iran.

The U.S. Central Command said that the U.S. blockade, starting at 10 a.m. ET on Monday (1400 GMT), would be "enforced impartially against vessels of all nations entering or departing Iranian ports and coastal areas, including all Iranian ports on the Arabian Gulf and Gulf of Oman."

Vessels transiting the Strait of Hormuz to and from non-Iranian ports will not be impeded, the U.S. military said.

President Donald Trump said on Sunday U.S. forces would also intercept every vessel in international waters that had paid a toll to Iran.

"No one who pays an illegal toll will have safe passage on the high seas," Trump wrote on social media, adding: "Any Iranian who fires at us, or at peaceful vessels, will be BLOWN TO HELL!"

-Reporting by Sam Tabahriti, Editing by Paul Sandle and Michael Holden/Reuters