WASHINGTON - World powers resumed talks Tuesday in Vienna about revitalizing the 2015 international pact to restrain Iran's nuclear development program and bring the United States back into the accord that former U.S. President Donald Trump unilaterally withdrew from in 2018.

Current U.S. President Joe Biden wants to rejoin the pact aimed at keeping Tehran from developing nuclear weapons, although Iran says its nuclear program is for peaceful purposes.

The U.S. does not have a seat at the table for the negotiations, but diplomats from the other countries in the agreement - Russia, China, Britain, Germany and France - are representing its interests at the talks.

Trump, believing the agreement approved by former U.S. President Barack Obama was too weak to prevent Iran from developing a nuclear weapon, instead stiffened U.S. economic sanctions against Tehran in hopes it would agree to tougher restrictions on its nuclear program. But no new agreement was reached before Trump left office in January.

Tehran, with the country's economy reeling, instead has steadily increased the purity of uranium it enriches and its stockpiles in a so-far unsuccessful effort to get the other countries in the pact to provide economic relief.

Iranian Foreign Minister Mohammad Javad Zarif speaks on the second day of the Munich Security Conference in Munich, Germany, Feb. 15, 2020.

The new talks are occurring days after comments surfaced from Iranian Foreign Minister Mohammad Javad Zarif alleging that Russia at one point tried to undermine the pact, presumably in hopes that continued U.S.-Iranian hostility would deflect American pressure on Moscow.

The Russian Foreign Ministry has yet to address Zarif's comments, made in a seven-hour interview with a research group associated with the Iranian presidency.

Before the talks began, Russian representative Mikhail Ulyanov said he had met with officials from Iran and China but did not say anything about Zarif's comments.

"We compared notes and exchanged views on the way ahead towards full restoration of the nuclear deal," he tweeted. "It was a very fruitful meeting."

Ulyanov later said the main negotiations were "guided by the unity of purpose."

The focus of the new talks is on the extent to which the U.S. would ease its economic sanctions and how Iran would again comply with the terms of the 2015 deal.