VIENNA -- Iran's Supreme Leader Ayatollah Ali Khamenei said on Tuesday the war in Ukraine should be stopped and accused the "mafia-like regime" of the United States of creating the conflict.

Russia, whose troops invaded Ukraine last week, is a strategic partner for Iran, which has been under Western sanctions for years. While Tehran and Washington have been foes for decades, Iran and Russia have deepened trade ties and have been allies in the Syrian conflict.

"The U.S. regime creates crises, lives off of crises and feeds on various crises in the world. Ukraine is another victim of this policy," Khamenei said in a televised speech.

"In my view, Ukraine is a victim of the crises concocted by the United States," he said. "There are two lessons to be learnt here. States which depend on the support of the U.S. and Western powers need to know they cannot trust such countries."

Khamenei criticized Washington and other Western nations as talks reached a critical stage in Vienna between Iran and world powers about reviving a 2015 nuclear deal.

Despite progress in the talks, the key sticking point is Tehran wants the issue of uranium traces found at several old but undeclared sites in Iran to be dropped and closed forever, an Iranian official told Reuters.

Tehran objects to claims by the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), that Iran failed to fully explain the uranium traces.

Britain's lead negotiator at the talks, Stephanie Al-Qaq, defended the IAEA, and said Britain, France, and Germany opposed interfering in its work.

"We will always reject any attempt to compromise IAEA independence," she wrote on Twitter.

Iran said on Monday efforts to revive the pact could succeed if the United States took a political decision to meet Tehran's remaining demands, as months of negotiations enter what one Iranian diplomat called a "now or never" stage.

The stakes are high, because the failure of 10 months of talks could carry the risk of a fresh regional war, more harsh sanctions on Iran by the West, and continued upward pressure on world oil prices already strained by the Ukraine conflict.

Iran's Foreign Ministry spokesman has said the remaining issues included the extent to which sanctions would be rolled back and providing guarantees that the United States would not quit the pact again.

All parties involved in the talks say progress has been made toward the restoration of the pact to curb Tehran's nuclear program in exchange for sanctions relief, which the United States abandoned in 2018. But both Tehran and Washington have said there are still some significant differences to overcome.

(Reporting by Dubai Newsroom; Writing by Michael Georgy and Arshad Mohammed; Editing by Alex Richardson, Jon Boyle and Rosalba O'Brien)