The United States has accused UN Secretary-General António Guterres of giving in to Russian threats and not sending officials to Ukraine to check the drones used by Russia, which, according to Washington, were supplied by Iran, the Voice of America has reported.

Russia denies that its armed forces used Iranian drones in Ukraine and claims that UN representatives have no mandate to travel to Kiev to investigate the origin of the drones.

Tehran acknowledged that it had supplied Moscow with the drones, but explained that they were sent before Russia invaded Ukraine in February.

Britain, France, Germany, the United States and Ukraine insist that Iranian drone deliveries to Russia violate a 2015 UN Security Council resolution.

These countries want Guterres to send UN representatives to Kiev to investigate.

"We regret that the U.N. has not moved to carry out a normal investigation of this reported violation," the U.S. representative to the UN said.

At Monday's U.N. Security Council meeting on the 2015 Iran nuclear deal resolution, Deputy U.S. Ambassador to the U.N. Ambassador Robert Wood:

"We are disappointed that the Secretariat, apparently yielding to Russian threats, has not carried out the investigatory mandate this council has given it," he said.

Earlier in his report to the Council, Guterres said U.N. officials were reviewing the information available and any findings would be reported in due course.

Asked Monday about the pressure he faced, Guterres explained to reporters that accusations that Iran had supplied Russia with drones used in Ukraine were being considered "in the broader picture of everything we are doing in the context of the war to determine if and when we should."

Russia's permanent representative to the UN, Vasily Nebenzia, told the Security Council that UN representatives must not yield to pressure from Western countries and that any results of this pseudo-investigation are invalid.

Iran's permanent representative to the UN, Amir Saeed Iravani, said that Iran had not transferred any items banned by the Security Council to Russia.

He also added that Iranian drones delivered to Russia before February were not banned by the Security Council and were not transferred for use in the ongoing conflict in Ukraine.