In a speech to a group of teachers in Tehran on Wednesday, Iranian Supreme Leader Ayatollah Ali Khamenei accused U.S. officials of threatening a military attack against his country, something he warned could torpedo the proposed nuclear deal, according to The Wall Street Journal.

Khamenei gave no details about the purported threat.

"Recently two U.S. officials threatened to take military action against Iran. What does negotiation mean under the shadow of threat," Khamenei said, according to Iran's state-run media, the International Business Times reports.

"Our negotiators should continue the talks with respect to our redlines," Khamenei said. "They should not accept any imposition, humiliation, and threat. Holding nuclear talks [with P5+1 powers] under shadow of threat is unacceptable for Iran."

"The United States is no less in need of these talks than Iran."

It's not the first time Khamenei has wavered on his support for a deal with the P5+1 powers — the U.S., Britain, China, France, Russia and Germany — that reportedly would ease economic sanctions on Tehran in return for strict limits on its nuclear sites, centrifuges that can enrich uranium, and enriched uranium stockpiles.

Last month, he said he "neither backs nor rejects" the framework deal reached in early April in Switzerland following months of negotiations, according to The Guardian.

Khamenei characterized the framework deal as "non-binding and insisted there was no guarantee a final deal would be struck."

Negotiations are scheduled to resume in Vienna on May 12. There is a June 30 deadline for a comprehensive agreement.

There have been tensions over Iran's seizure of the Maersk Tigris, a cargo ship with a Marshall Islands flag, on April 28. Iranian officials maintain that the ship entered Iranian waters in the Strait of Hormuz, an area described by the Journal as a 21-mile-wide Persian Gulf outlet through which 30 percent of the world's seaborne oil shipments pass.

Iranian officials have said the ship would be released after fines that resulted from a legal dispute are paid. Since the incident, the U.S. military has begun escorting American ships through the strait.

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