WASHINGTON — The next round of nuclear talks between Iran and the major powers will attempt to “bridge the gaps” toward a comprehensive agreement, the top European negotiator said.

“A lot of intensive work will be required to overcome the differences ahead,” Catherine Ashton, the top European Union foreign policy official, said Wednesday at a news conference summing up two days of talks in Vienna between Iran and the six powers — United States, Germany, France, Britain, Russia and China.

“We will now move to the next phase in the negotiations in which we will aim to bridge the gaps in all the key areas and work on the concrete elements of a possible comprehensive agreement,” she said.

The next talks are scheduled to take place in the Austrian capital over two days starting May 13.

Javad Zarif, the Iranian foreign minister, addressing the same news conference, read the same statement in his native Farsi but added that he believed the sides were in “50-60 percent agreement” on the issues.

The deadline for such a deal is the end of July, although the talks may then be extended.

A senior US administration official declined to confirm whether Zarif’s assessment was accurate but said only 100 percent agreement by both sides would produce a deal. She demanded anonymity in line with State Department briefing rules.

Unlike previous rounds the May meeting will be open-ended to allow negotiators to meet all week if needed as they increase efforts to seal a deal, she said.

The world powers are offering to remove sanctions that have crippled Iran’s economy provided Tehran agrees to strict long-term limits on any nuclear activities that could be used to make a weapon.

The future scope of Iran’s uranium enrichment program is the toughest issue.

Iran's Supreme Leader Ayatollah Ali Khamenei delivers a speech in Tehran (photo credit: AP/Office of the Iranian Supreme Leader/File)

Iran’s Supreme Leader Ayatollah Ali Khamenei delivers a speech in Tehran (photo credit: AP/Office of the Iranian Supreme Leader/File)

Iran denies any interest in nuclear arms and argues it needs robust enrichment capacities to make low-enriched reactor fuel. The US, Britain, France and Germany want significant cuts, to decrease the chances that the program will be re-engineered to make high-enriched material for atomic arms. Russia and China are somewhere in the middle.

The six also want to eliminate potential proliferation dangers from an enrichment site at Fordo, south of Tehran, that is built far underground to withstand air strikes, and at a nearly built nuclear reactor at Arak, in northwestern Iran, that could produce substantial amounts of plutonium unless it is changed to a model with new specifications.

Like enriched uranium, plutonium can be used for the core of a nuclear weapon.

Russian chief negotiator Sergei Ryabkov noted progress on the reactor issue, but said no specific plan had been put forward.

“We are advancing centimeter by centimeter, drop by drop,” he told the Interfax news agency.

A first-step agreement, in effect since January, has curbed some Iranian nuclear activities in return for limited sanctions relief as the two sides work toward a final accord. Iranian hardliners fear that end deal will cut too deeply into their country’s nuclear program, a source of national pride.

In Tehran, Iran’s supreme leader Ayatollah Ali Khamenei sought to dispel such fears.

“These talks need to continue,” he said. “But all must know that despite continuation of the talks, activities of the Islamic Republic of Iran in the field of nuclear research and development won’t be halted at all.”

Speaking to a group of Iranian nuclear scientists, he said, “Our negotiators should not accept any coercive words from the other party.” And added, “The country’s nuclear achievements can’t be stopped, and no one has the right to bargain over it.”

On Tuesday, US Secretary of State John Kerry faced blunt questions from the Senate on the talks, including repeated reminders that Congress must approve multiple aspects of any deal, especially those concerning sanctions rollbacks.

Sen. Robert Menendez (D-NJ), the chairman of the Senate Foreign Relations Committee, expressed concern at reports that the United States would settle for a “breakout” for Iran of six to 12 months, referring to the period in which Iran could create enough fissile material for a nuclear device.

“A deal that would ultimately unravel the entire sanctions regime for a six- to 12-month lead time is not far from where we are today,” Menendez said. “I’m trying to get a sense of these parameters, because to the extent that the administration has asked for forbearance, part of it is going to have to be based on having an understanding of what is the parameters.”

Undated photo of retired-FBI agent Robert Levinson (photo credit: AP/Levinson Family)

Undated photo of CIA contractor and retired-FBI agent Robert Levinson (photo credit: AP/Levinson Family)

Kerry said that Iran’s breakout time now stood at two months, and that six to 12 months was “significantly more” than that, but adding that the United States would not necessarily settle for even the longer period. He said the key component of the agreement would be rigorous monitoring.

“The greater likelihood is that at the end of this, we hope to be able to come to you with an agreement that has the most extensive and comprehensive and accountable verification process that can be achieved in order to know what they’re doing,” Kerry said.

He also said that in every meeting with Iranian officials, US officials raise the status of three Americans held in Iran, including Robert Levinson, a retired FBI agent who is Jewish.