"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.
He added that President Barack Obama "directed his team to remain engaged and participate in conversations as long as those conversations remain useful."
The White House spokesman also suggested that the negotiations in Vienna could go into Tuesday.
Earlier, Iranian Foreign Minister Mohammad Javad Zarif said it is “possible” that the talks will come to a conclusion on Tuesday.
Zarif made the remarks from the balcony of Palais Coburg hotel, the venue for the negotiations, in response to a question asked by Press TV reporter Homa Lezgee, who is in Vienna to cover the talks.
The minister also tweeted: "If #IranDeal reached, triumph of diplomacy means we all will have won when we all could have lost. Plain and simple; no spin needed."
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