U.S. Secretary of State John Kerry called Prime Minister Benjamin Netanyahu on Monday evening to update him ahead of the decisive round of talks with Iran opening Tuesday in Vienna.

A senior U.S. administration official said in a press briefing ahead of the talks that a permanent agreement could still be reached with Iran on its nuclear program before the end of the talks on November 24. “It will be tough, but it’s possible,” the senior official said.

The negotiating teams representing Iran and the six powers – the United States, Russia, China, Britain, France and Germany – are to arrive in Vienna on Tuesday and begin talks in the afternoon. Kerry is expected to arrive in Vienna toward the end of the week to follow the talks closely. The other foreign ministers may also join the talks if needed.

A senior American official said that efforts to reach an agreement would continue until the last minute and that some progress had been achieved in talks last week both in Muscat, Oman and during expert-level meeting since then, but that gaps do remain. “We still don’t know if we can do it,” the officials said.

Iranian and Western diplomats have been saying over the past few days that an agreement could not be reached by November 24 and there would be no alternative but to extend the talks. The assessment in the Israeli intelligence community and the Foreign Ministry in Jerusalem is also that the parties would extend the talks by a few months.

According to a senior official, Israel believes that Iran and the international powers will try to reach an agreement on issues were consensus has already been reached and would extend talks on the other matters for a few months.

“The gaps are very, very large,” said a senior Israeli official familiar with the status of the talks between Iran and the international powers. “If there is no dramatic movement of one of the sides, there will be no agreement,” he added.

At the press briefing, the senior U.S. official rejected reports of a possible extension of talks, and said that the ball was in the Iranian court in terms of a breakthrough. The official said Iran had to back up its words with actions to provide the international community with guarantees that its nuclear program was for peaceful uses only. “We knew that the difficult decisions would not be made until the end – if at all,” he said, adding “we won’t know how far we can get until we reach November 24.”

Netanyahu is anxiously watching the round of talks to open on Tuesday. He is continuing to operate vis a vis the U.S. Congress and public opinion against a deal with Iran. On Sunday, he gave interviews to American morning TV shows and warned against a bad deal with Iran just around the corner.

The senior U.S. official said that he understands Netanyahu’s concern, and that “If President Obama believes we can reach a deal, that means it’s a good deal.”

According to the official, Obama considers it very important that the deal with Iran not harm Israel’s security and demands that no agreement with Iran leave any channel through which Iran can produce nuclear weapons. “Prime Minister Netanyahu will have to judge the agreement if and when we achieve it,” the senior official said.

In talks over the past few months, representatives of Iran and the six powers reached understandings on a number of subjects. For example, Iran agreed to make technical changes in the heavy water reactor in Arak so as to limit to a very low level its capability to produce plutonium. The Iranians also agreed to commit to build a facility to convert plutonium into a material from which nuclear weapons can be made.

Over the course of the talks, the Iranians have refused to close the fortified underground enrichment plant at Pardo; however, they have reached understandings with the six powers that the facility there would become a research and development center and would be monitored closely by the International Atomic Energy Agency.

Another issue in which progress has been made involves Iran’s reserve of enriched uranium. Iran now has more than eight tons of uranium enriched to 5 percent. The six powers are willing to leave Iran a symbolic 350 kilograms, which is less than the quantity needed to produce one atomic bomb. The powers have proposed that the rest of the material be sent to Russia to be converted into nuclear fuel for the nuclear power station at Bushehr, and the Iranians tend to agree to this proposal.

However, the parties still disagree on three core issues of the talks. The Iranians will not give up their 19,000 centrifuges and are demanding to expand their enrichment array. The powers are willing to allow them to keep about 4,500 centrifuges only, and are demanding that the rest be neutralized and critical components given to IAEA monitors.

Another bone of contention is the sanctions: The Iranians want all sanctions against them lifted as soon as the agreement is signed, while the powers are proposing that the sanctions be lifted gradually over a few years in keeping with the agreement’s implementation. Another remaining disagreement surrounds how long the deal will remain in place: The power are demanding it stay in effect 10 to 15 years, while the Iranians want the accord to be limited to a period of five to seven years.

