But spokeswoman Marie Harf added that "several difficult issues" still remained.
World powers want to restrict Iran`s nuclear programme in return for relief from crippling sanctions.
Earlier, there were suggestions that a general statement would be released focusing on points of agreement, which would be enough to allow for a new phase of talks on a comprehensive deal.
But there is no sign that the most contentious issues have been resolved.
"The negotiations will end when solutions have been found," said Iranian negotiator Hamid Baidinejad. "We are not watching the clock."
The so-called P5+1 - the US, UK, France, China and Russia plus Germany - are seeking to ensure Iran could not assemble a nuclear weapon in less than a year.
The Iranians insist that they have no such ambition.
`Good` chance
Foreign ministers met several times at Lausanne`s Beau-Rivage Palace hotel on Tuesday morning in a high-level push to meet the self-imposed deadline to agree a political framework agreement by the end of the day.
Russia`s top diplomat, Sergei Lavrov, announced that prospects were "very good and promising", and said he was returning to the talks after leaving for another engagement on Monday.
The deadline extension signifies the difficulty of reaching agreement on some of the outstanding issues, but also the prospect of achieving some kind of deal.
Officials told the Associated Press that there were plans to issue a "framework understanding", accompanied by documents outlining more technical details.
This would be enough for all parties to continue negotiations in a new phase aimed at achieving a comprehensive accord by 30 June, our correspondent says.
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