WASHINGTON--International negotiations aimed at reaching a nuclear agreement with Iran could be extended past the Tuesday deadline, U.K. Foreign Secretary Philip Hammond said Friday.

Mr. Hammond, visiting Washington, said the talks could last as much as two weeks longer, coinciding with the U.S. congressional recess. Lawmakers have been delaying action on legislation to ramp up punitive sanctions against Iran in the absence of a deal by the deadline, but the two-week congressional break provides negotiators meeting in Lausanne, Switzerland, with more time, Mr. Hammond said.

"I hope that doesn't mean we have to sit in Lausanne for another two weeks just because there is another two weeks," Mr. Hammond said. "It means we have got a little bit more negotiating time if we need it, but I think we're all hopeful of getting it done as soon as possible."

The March 31 deadline was a nonbinding target set after negotiators were unable to meet a Nov. 24, 2014, deadline. Negotiators had planned to hammer out a "framework" for an agreement by the end of this month, with technical details due by the end of June. Other officials have said the deadline could slip, but Mr. Hammond's remarks came during what some diplomats have described as the final round of talks.

Mr. Hammond didn't rule out the possibility of an agreement by the Tuesday deadline.

"We're hopeful that we'll be making that progress over the next 48 hours, " Mr. Hammond said. "Of course we'd like it to happen sooner, but if it takes longer, we'll keep at it, we'll stick at it."

Secretary of State John Kerry is in Lausanne now, but Mr. Hammond said he would wait to return to the talks until technical differences have been settled and top international officials can focus on "big-picture issues." That said, Mr. Hammond added that he could fly to Switzerland as early as this weekend. Officials expect him to arrive Sunday.

There are six "key areas of discussion" that await resolution before reaching a framework accord, Mr. Hammond said.

"Some of them we are close enough that we can be sure we'll reach an agreement," he said. "There are still some areas where we are significantly apart. It's going to require a significant move by the Iranians."

He added, "We're better than halfway in terms of having numbers of areas where we are close enough that we can be confident in the endgame."

As the talks continue, Britain is concerned about events in Yemen, which increasingly have taken on the appearance of a proxy war involving Iran and Saudi Arabia. Mr. Hammond said attempts are under way to encourage parties in Yemen to return to a political process.

Britain hopes both Yemen and the Saudis will cooperate in those efforts, he said.

Mr. Hammond was in Washington to speak at the State Department to an annual meeting of U.S. ambassadors. He is meeting Friday with Sen. Bob Corker (R., Tenn.), chairman of the Senate Foreign Relations Committee, and National Security Adviser Susan Rice. Earlier Friday, he met with retired Gen. John Allen, the Obama administration's envoy to the effort to battle Islamic State militants.

Write to Felicia Schwartz at Felicia.Schwartz@wsj.com

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