U.S. Secretary of State John Kerry has pushed back a visit to Vienna, where international negotiators are trying to seal a deal on Iran's nuclear program ahead of a commonly agreed November 24 deadline.
Officials have said substantial differences remain between Iran and Western powers after months of negotiations to build on an interim deal agreed last November and end a 12-year-old standoff.
Kerry had been expected to arrive in Vienna in the middle of the week as part of the push for a deal to curb Tehran's nuclear activities, which Western nations fear could lead to the development of atomic weapons, and phase out UN and Western economic sanctions that have hurt Iran's oil-reliant economy.
But State Department spokeswoman Jen Psaki said late on November 18 that Kerry would travel to Vienna "later in the week," although the exact day has yet to be decided.
Psaki said Kerry would remain in London on November 19 and travel to Paris on the morning of November 20.
While in London, Psaki said, Kerry would "continue consulting with both the negotiating team in Vienna" and with U.S. officials in Washington.
In Paris, he will discuss the Iran negotiations in separate meetings with the Saudi and French foreign ministers.
Iran's Foreign Minister Mohammad Javad Zarif said upon arrival in Vienna on November 18 that an agreement was "possible."
But he cautioned that if the talks fail, it will be because the six nations negotiating with Iran -- the United States, Russia, China, Britain, France, and Germany -- asked for too much.
Zarif told Iranian media, "If, because of excessive demands... we don't get a result, then the world will understand that the Islamic republic sought a solution, a compromise, and a constructive agreement and that it will not renounce its rights and the greatness of the nation."
British Foreign Secretary Philip Hammond said on November 18 that it is Iran that must show more "flexibility" if a deal is to be reached.
The talks have been stalled for months over Tehran's refusal to consistently reduce the size and output of centrifuges that can enrich uranium both to levels needed for reactor fuel or the core of nuclear warheads.
International sanctions have harmed the oil-dependent Iranian economy and inflation and unemployment have been soaring.
Iran denies Western accusations that its nuclear program aims secretly at developing a nuclear weapon, saying it is meant only for peaceful purposes such as power generation.
But a report on November 7 by the International Atomic Energy Agency (IAEA) said Iran was failing to address the accusations.
Ambassador Laura Kennedy, the U.S. envoy to the Vienna-based IAEA, said November 17 that she would convey "our concerns with Iran's failure to engage substantively with the agency on the possible military dimensions issue" in a statement to the IAEA's 35-nation Board of Governors, which convenes on November 20.