US Secretary of State John Kerry has pushed back his trip to Vienna where international negotiators are in final talks on Iran's nuclear programme, his department said.
Kerry, who has been in London since Monday evening, was due to arrive mid-week in the Austrian capital to complete negotiations with the "5+1" group (United States, Russia, China, France, Britain and Germany) and Tehran.
Leaders hope to seal a landmark agreement by the November 24 deadline after months of negotiations but substantial differences remain between Iran and Western powers.
Iranian Foreign Minister Mohammad Javad Zarif and Europe's chief negotiator Catherine Ashton resumed discussions Tuesday in Vienna, along with representatives of 5+1, ahead of the arrival of their six foreign ministers.
But Kerry will now travel to Vienna "later in the week", state department spokeswoman Jennifer Psaki indicated, adding that the exact day had not yet been set.
"Secretary Kerry will stay in London tomorrow where he will continue consulting with both the negotiating team in Vienna and his interagency counterparts in Washington," she said late Tuesday.
"He will travel to Paris on Thursday morning where he will have separate meetings with Saudi Foreign Minister Saud and French foreign Minister Fabius on the Iran nuclear negotiations."
Speaking earlier in London Kerry said it was "imperative that Iran works with us with all possible effort to prove to the world that the programme is peaceful".
"This is a very critical week obviously in Iran negotiations," he added. "We hope we get there but we can't make any predictions."