UPDATE: Nuclear talks between world powers and Iran will continue Thursday, after being extended twice past today’s deadline.

Talks on Iran’s nuclear program were extended on Wednesday after overrunning the March 31 deadline. U.S. Secretary of the State John Kerry said that he expected to reach an agreement by Wednesday evening. Major powers (the U.S., UK, Russia, China, France and Germany) and Iran had moved closer to a preliminary accord on Iran’s disputed nuclear development program as marathon talks ran into Wednesday.

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Iran nuclear talks: deadlock over key details

But the group of international negotiators and Iran hit an impasse over crucial details such as lifting of the U.N. sanctions, Tehran’s nuclear centrifuge research, and restoration of sanctions if Iran breached the agreement. John Kerry told reporters in the Swiss city of Lausanne that he hoped to hammer out an accord by Wednesday evening.

Earlier today, German Chancellor Angela Merkel said most of the steps towards a deal have been made. World powers have been negotiating a deal with Iran to stop its uranium enrichment program. Tehran says that its nuclear program is for civilian energy, but major powers believe it could be developing nuclear weapons.

An immediate deal unlikely, says France

The preliminary accord would be a major milestone towards the final accord, which has a June 30 deadline. The deal would end over a decade-long standoff between world powers and Iran, and alleviate the risk of a Middle-East war. French foreign minister Laurent Fabius returned to Paris, saying he would come back only if it was necessary. He said the negotiators have made progress, but not enough to reach an immediate deal.

However, Russia and Iran expressed optimism that a preliminary accord was still within reach. Russian foreign minister Sergei Lavrov said negotiators had reached an accord on “all key aspects.” Chinese diplomats warned of failure, asking all sides to meet each other half way. On Tuesday, Iran reaffirmed its “nuclear rights.” Officials worry that any agreement would be incomplete and fragile.

Iran’s deputy foreign minister Abbas Araqchi said the negotiators are expected to make a joint statement on the progress later on Wednesday.