The White House said on Thursday that Iranians have been on the ground in Crimea helping Russians target Ukraine with Iranian drones, and that U.S. efforts to negotiate a return to the Iran nuclear deal have been set aside for the time being.

John Kirby, the White House's top national security spokesperson, told reporters that Russian military personnel, operating remotely from Crimea, have been piloting Iranian drones to launch attacks against Ukrainian targets including Kyiv in recent days.

Russia seized Crimea from Ukraine in 2014.

"We assess that Iranian military personnel were on the ground in Crimea and assisted Russia in these operations," Kirby said. He added that it appeared a relatively small number of Iranians were involved.

There was no immediate public reaction to the U.S. allegations from Tehran. Russia's defense and foreign ministries did not immediately respond to requests for comment.

The disclosure marked a fresh point of tension between the United States and Iran, whose crackdown on protesters following the death in September of a woman who was arrested in Tehran for "inappropriate attire" has been condemned by President Joe Biden.

Kirby indicated that the United States saw no need at this point to continue trying to negotiate with Tehran over Iran's return to the Iran nuclear deal with six world powers in 2015.

Biden had sought a return to the deal to provide international monitoring of Iran's nuclear program in return for a lifting of sanctions against Tehran. Biden's predecessor, Donald Trump, had abandoned the agreement, known as the Joint Comprehensive Plan of Action, in 2018.

"We are way far apart with the Iranians in terms of a return to the deal, so we are simply not focused on that right now. They had demands that were well in excess what the JCPOA was supposed to cover. We are not focused on the diplomacy at this point," he said.

Less than two months ago U.S. officials reported some progress in reviving the deal.

The United States is now concerned that Russia may seek to acquire advanced conventional weapons from Iran such as surface-to-surface missiles for use against Ukraine, Kirby said.

He said the United States is going to pursue all means to "expose, deter and confront" Iran's supply of munitions to Russia, including more sanctions.

"Iran and Russia, they can lie to the world, but they certainly can’t hide the facts, and the fact is this: Tehran is now directly engaged on the ground," he said.