U.S. Secretary of State John Kerry addresses the media during a news conference in Vienna July 15, 2014. Kerry on Tuesday dismissed the idea that Iran could maintain its current number of nuclear enrichment centrifuges as part of a long-term deal with six world powers that would lead to a gradual end of sanctions. (Heinz-Peter Bader/Reuters)

The Obama administration is reassessing whether a nuclear deal with Iran is possible despite wide differences over key issues and may seek to postpone a deadline looming Sunday to either complete an agreement or walk away from the landmark effort.

Secretary of State John F. Kerry on Tuesday cited progress in intensive negotiations in Vienna, shortly before returning to Washington to consult with the White House and Congress about what to do next. But he told reporters that “very real gaps” remain in the talks and that officials must decide whether an extension of talks is prudent, based on progress to date.

After a diplomatic breakthrough last year that installed temporary curbs on Iran’s disputed nuclear program, six months of talks have failed to bridge a gulf over the most basic question of how much of its program Iran could retain in exchange for a lessening of crushing international sanctions.

“It is clear that we still have more work to do,” Kerry said.

Critics of the negotiations pounced. “Iran’s supreme leader has made clear that Iran will not agree to dismantle its nuclear weapons program. In fact, he seeks to expand it,” said Rep. Edward R. Royce (R-Calif.), chairman of the House Foreign Affairs Committee.

Extending the July 20 deadline for talks would require agreement among Iran and the six world powers seeking a deal after years of mostly fruitless negotiations. For the Obama administration, extending the deadline would also reopen contentious debate with Republicans and some Democrats in Congress over whether the terms are too generous to Iran and potentially dangerous for Israel, the United States’ closest ally in the Middle East.

“In light of Secretary Kerry’s comments today in Vienna that ‘very real gaps’ remain between Iran and the international community, my hope is that the administration will finally engage in robust discussions with Congress about preparing additional sanctions against Iran,” Royce said Tuesday.

A bipartisan group of more than 340 House members wrote to President Obama last week to urge wider discussion with Congress about the potential agreement, while even Democrats close to the White House have warned of the dangers of a hasty deal.

Congress would have to approve the eventual lifting of onerous U.S. sanctions. Iran has sought the lifting of the sanctions, imposed as punishment principally for a uranium-enrichment program that the West says is far larger than required for peaceful energy and medical needs. The sanctions have severely hobbled Iran’s oil exports and restricted its imports of foreign goods.

Extending the talks could make the Obama administration look eager or could open the White House to charges that it is rolling over in the face of Iranian stalling.

Congress is already prepared to add more sanctions that backers say would increase U.S. leverage in the nuclear talks, and despite earlier White House success in holding off such legislation, it may be unable to do so now.

Iran’s supreme leader, Ayatollah Ali Khamenei, said last week that Tehran intends to expand its uranium-enrichment capacity and would need many more centrifuges than it now possesses. The U.S. goal in talks has been to reduce the number of centrifuges, as Kerry reiterated Tuesday.

Kerry had been reportedly planning to travel to the Middle East for discussions with Egyptian leaders over the conflict in the Gaza Strip, but the impasse over the nuclear talks with Iran required urgent discussions in Washington, U.S. officials said.

Sen. Robert Menendez (D-N.J.), chairman of the Senate Foreign Relations Committee, said Tuesday that an expanded Iranian capacity would make a deal impossible.

Iran and international negotiators brokered a temporary deal last fall that set boundaries for the nuclear program in exchange for limited relief from international sanctions on Iran. That deal expires Sunday unless it is replaced or extended.

Iranian Foreign Minister Mohammad Javad Zarif said he had held good talks with Kerry over the past two days, but he agreed that there is still a gulf.

“Washington needs to take a political decision . . . to end the deadlock,” Reuters quoted Zarif as saying in Vienna.