Iran advised its citizens on Friday not to travel to Ukraine and urged Iranians already there to leave, semi-official news agencies reported, a day after the United States accused Iran of helping Russia operate drones against Ukraine.

The Iranian foreign ministry advisory did not refer to the US allegation. It gave a telephone number for the Iranian embassy in Kyiv to call for advice.

“Due to the military escalation in Ukraine, all Iranians are strongly advised to refrain from travelling to Ukraine. Also, Iranians living in Ukraine are advised to leave the country for their own safety,” the ministry statement said.

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

Russia seized Crimea from Ukraine in 2014.

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, White House national security spokesperson John Kirby told reporters.