LAUSANNE, Switzerland -- Negotiations are intensifying in Switzerland as a deadline to reach a preliminary deal on Tehran's nuclear program looms, with differences still remaining.
Top diplomats from six world powers are meeting their Iranian counterpart in the Swiss resort city of Lausanne on March 30 for the talks.
EU foreign policy chief Federica Mogherini is also there.
The delegations were working under a self-imposed March 31 deadline for a framework agreement aimed at curbing Tehran’s nuclear activities in exchange for relief from economic sanctions.
The outline is meant to pave the way for a final agreement by the end of June.
The five permanent members of the UN Security Council -- Britain, China, France, Russia, and the United States -- plus Germany want to ensure that Iran cannot develop nuclear weapons.
Tehran denies it is secretly developing a nuclear weapon, saying it only wants to develop civilian nuclear power for domestic needs.
A Western diplomat said on March 30 it was unclear whether a framework deal can be reached by the self-imposed deadline.
The unidentified diplomat said the most difficult issues were related to the duration of any limits on Iranian nuclear activities after an initial 10 years, the lifting of UN sanctions against Iran, and the restoration of sanctions if Tehran fails to comply.
"There cannot be an agreement if we do not have answers to these questions," Reuters news agency quoted the diplomat as saying.
"The feeling is that if things are to happen, it's now that the pieces will fit together,” he added. “There's a moment when you have to say yes or no."
U.S. Secretary of State John Kerry told a CNN reporter that he and his counterparts would be working “late into the night and obviously into tomorrow” to resolve "tricky issues" blocking a deal.
Acting State Department spokeswoman Marie Harf appealed to the negotiators, saying "now is really the time to make the decisions."
She also said Washington was not going to "rush to make a bad deal."
Chinese Foreign Minister Wang Yi said "Positions are narrowing," adding that he was "cautiously optimistic."
German Foreign Minister Frank-Walter Steinmeier said there had been "some progress and some setbacks in the last hours."
Russia's chief negotiator, Deputy Foreign Minister Sergei Ryabkov, said the discussions were "extremely intensive."
He said, "The main thing that causes optimism is determination of all ministers to achieve results...within the current session."
Russian Foreign Minister Sergei Lavrov left the talks on March 30 to return to Moscow. Lavrov's spokeswoman, Maria Zarakhova, said he would return to Lausanne on March 31 if there is a realistic chance for a deal.
"I'm not going to presuppose failure,” White House spokesman Eric Schultz told reporters. “Those negotiations are going to go down to the wire."
Israel, which is not a party to the talks, has again warned of a nuclear deal with Iran.
Referring to Iran’s alleged support for Shi’ite rebels in Yemen, Israeli Prime Minister Benjamin Netanyahu said, "The agreement being formulated in Lausanne sends a message that there is no price for aggression and on the contrary -- that Iran's aggression is to be rewarded."
"But we are not closing our eyes and we will continue to act against every threat in every generation, certainly in this generation," Netanyahu said in a statement.