Prime Minister Theresa May has reaffirmed Britain's commitment to a 2015 Iran nuclear deal 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 program in return for lifting most Western economic sanctions.A senior US administration official said last week that Trump - who has criticized 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," said a statement from May's office following the call on Tuesday evening."(The prime minister) stressed that it was important that the deal was carefully monitored and properly enforced."In contrast, a White House statement on the phone call said Trump "underscored the need to work together to hold the Iranian regime accountable for its malign and destabilizing activities, especially its sponsorship of terrorism and its development of threatening missiles."In a separate statement, Britain's Foreign Office said Iran had upheld its nuclear commitments, adding to international pressure on Trump not to jeopardize security in the region."The nuclear deal was a crucial agreement that neutralized Iran’s nuclear threat," foreign minister Boris Johnson said."It was the culmination of 13 years of painstaking diplomacy and has increased security, both in the region and in the UK. It is these security implications that we continue to encourage the US to consider."