UPDATE 5:20 p.m. EDT:

Iranian nuclear talks will not be allowed to continue past dawn Wednesday, the Iranian delegation was told, Reuters has reported. U.S. State Department and White House officials had indicated previously that the negotiations would continue Wednesday.

Original story:

Negotiators trying to reach a nuclear deal with Iran were poised to continue working past a midnight deadline as their discussions stretched late into Tuesday night in Lausanne, Switzerland. The talks were expected to continue Wednesday, "as long as the conversations continue to be productive," White House press secretary Josh Earnest said. 

"We've made enough progress in the last days to merit staying until Wednesday," State Department spokeswoman Marie Harf said. "There are several difficult issues still remaining." The deadline for a draft agreement, a precursor to a comprehensive nuclear deal with Iran that those involved in the talks had hoped to reach in June, was a self-imposed one. 

The U.S., U.K., France, Russia, China and Germany -- often referred to as the P5+1 -- sought to ensure in the negotiations that Iran would be unable to manufacture a nuclear bomb within a year. Iran has insisted that it wants not the capacity to make nuclear weapons but the ability to produce nuclear power, and it has sought to have international sanctions imposed by the other countries lifted.

Foreign ministers of the P5+1 countries and Iranian Foreign Minister Mohammad Javad Zarif have been meeting this week in Lausanne for the first time since November. Also present for the negotiations were European Union foreign policy head Federica Mogherini and Abbas Araghchi, Iran’s lead negotiator.

In a move that initially appeared to bode well for the talks, Russian Foreign Minister Sergei Lavrov rejoined the negotiations Tuesday after leaving them Monday and saying he would return only if reaching an agreement appeared realistic. “I believe that the prospects are very good and promising,” Lavrov said Tuesday in a news conference. 

But reaching a deal required agreement on several key, persistent issues, including the number of centrifuges -- machines that enrich uranium -- that Iran would be allowed to have, along with other restrictions on its nuclear program. Another snag was the question of how quickly sanctions on Iran would be lifted, and even well before the deadline approached, it appeared that any agreement would lack specifics on these tougher issues. “Solutions have been found for numerous questions. We are still working on two or three issues,” Araghchi said Monday, the Agence France-Presse reported.

For 18 years, Iran hid the fact that it was enriching uranium, which can be used to produce a nuclear weapon. In 2013, the country agreed to a temporary deal that it would suspend parts of its nuclear enrichment program if sanctions were partly lifted, but a final agreement over sanctions and Iran's nuclear program has yet to be reached. In this latest round of talks, deadlines for deals had previously been extended twice since November 2013, although negotiations over Iran’s nuclear program have gone on for a more than a decade.

Critics of the deal, including U.S. House Speaker John Boehner and Israeli Prime Minister Benjamin Netanyahu, have said a deal would fail to ensure that Iran would not have the ability to produce a bomb.