Britain, France and Germany launched a 30-day process to reimpose UN sanctions on Iran over its nuclear programme on Thursday, a step likely to stoke tensions two months after Israel and the United States bombed Iran, according to a letter sent by E3 to the UN Security Council seen by Reuters.

The trio, known as the E3, said in a statement they had decided to trigger the so-called snapback mechanism before they lose the ability in mid-October to restore sanctions on Tehran that were lifted under a 2015 nuclear accord with world powers.

They have held several rounds of talks with Iran since Israel and the United States struck its nuclear installations in mid-June, aiming to agree to defer the mechanism but they deemed that talks in Geneva on Tuesday did not yield sufficiently tangible commitments from Iran.

The United States, which was party to that deal, pulled out under President Donald Trump in 2018, and held failed indirect negotiations earlier this year with Tehran.

The E3, whose ministers informed US Secretary of State Marco Rubio of their decision on Wednesday, said they hoped that Iran would engage by the end of September to provide commitments over its nuclear programme that will convince them to defer concrete action.

Iran has previously warned of a "harsh response" if sanctions are reinstated.

The E3 had offered to delay the snapback for as much as six months to enable serious negotiations if Iran resumes full UN inspections - which would also seek to account for Iran's large stock of enriched uranium that has not been verified since the June strikes - and engages in talks with the United States.