Marathon talks on a historic nuclear deal with Iran have made "real progress" but issues remain, the White House said, suggesting negotiations in Vienna would stretch into another day.
"They have made genuine progress... but there continues to be some sticking points that remain unresolved," White House spokesman Josh Earnest told reporters in Washington on Monday.
"The expectation right now is that the discussions will continue. I am not aware of any plan to take a break."
Earnest said the US and its partners did not want to rush the final stages of the lengthy talks.
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"Typically, some of the most difficult issues are the ones that get kicked to the end, and that's why the president is going to resist any effort to sort of fast-forward through the closing here," Earnest said.
Iranian Foreign Minister Mohammad Javad Zarif says no nuclear agreement will be signed on Monday.
Disputes persisted over attempts to probe Iran's alleged work on nuclear weapons, diplomats told AFP news agency, threatening plans to wrap up an international agreement by midnight - the latest in a series of deadlines for the negotiations.
The diplomats said at least two other issues still needed final agreement: Iranian demands that a UN arms embargo be lifted and that any UN Security Council resolution approving the broader deal no longer describe Iran's nuclear activities as illegal.
The diplomats spoke only on condition of anonymity.
Iran and six world powers are close to signing a historic nuclear deal that will bring sanctions relief in exchange for curbs on Tehran's atomic programme, officials say.
US Secretary of State John Kerry has cautioned that "major issues" remain to be resolved, and comments from both senior Republican and Democrat Senators on Sunday suggested that any final deal would also face tough scrutiny in the US Congress.
Iranian President Hassan Rouhani said on Sunday further measures were still needed to overcome the remaining obstacles in the Vienna negotiations, ISNA news agency reported.
"It might seem we have reached the top of the mountain. But no, there are still steps needed to be taken," ISNA quoted him as saying. "Even if we fail ... we have performed our duty."
Among the biggest sticking points in the past week has been Iran's insistence that a United Nations Security Council arms embargo and ban on its ballistic missile programme dating from 2006 be lifted immediately if an agreement is reached.
Russia, which sells weapons to Iran, has publicly supported Tehran on the issue.
Other problematic issues in the talks are access for inspectors to military sites in Iran, answers from Tehran over past activity and the overall speed of sanctions relief.
Kerry and Zarif have met nearly every day since Kerry arrived in Vienna more than two weeks ago for what was intended to be the final phase in a negotiation process that began with an interim nuclear deal clinched in November 2013.