LONDON (REUTERS) - Prime Minister Theresa May reaffirmed Britain's commitment to a 2015 Iran nuclear deal on Tuesday (Oct 10) in a telephone conversation with US President Donald Trump ahead of a key US decision on whether Tehran has stuck to the terms of the pact.

Trump has cast doubt on the future of the Joint Comprehensive Plan of Action (JCPOA), which sought to curb Iran's nuclear programme in return for lifting most Western economic sanctions.

A senior US administration official said last week that Trump - who has criticised the pact as an "embarrassment" and "the worst deal ever negotiated," - was expected to decertify Iran's compliance ahead of an Oct 15 deadline.

"The (Prime Minister) reaffirmed the UK's strong commitment to the deal alongside our European partners, saying it was vitally important for regional security," an emailed statement from May's office said.

"(The Prime Minister) stressed that it was important that the deal was carefully monitored and properly enforced."

They also discussed the need for Britain, the United States and others to work together to counter destabilising Iranian activity in the region, May's office said.

Britain and the United States are two of eight signatories to the deal, along with Iran, China, France, Russia, Germany and the European Union.

China, Russia and the European states have already expressed their continued support for the deal, while Iran has said Trump would not be able to undermine the pact.

If Trump declines to certify Iran's compliance, US congressional leaders would have 60 days to decide whether to reimpose sanctions on Tehran suspended under the agreement.

Earlier, the White House said Trump would make an announcement later this week on an "overall Iran strategy", including whether to decertify the nuclear deal.

May's office said she agreed with Trump that their teams should remain in contact ahead of the decision on recertification.