Iran and Russia on Tuesday condemned U.S. President Donald Trump's decision to maintain a military presence near oil fields in northeastern Syria, with Russian Foreign Minister Sergei Lavrov saying any exploitation of resources would be illegal.

Trump: U.S. troops won't immediately return from Syria

Trump confirms death of ISIL leader al-Baghdadi

Trump's suggestion on Sunday that Exxon Mobil Corp or another U.S. oil company operate Syrian oil fields drew rebukes from legal and energy experts.

The United States will strengthen its military presence in Syria with "mechanized forces" to prevent Islamic State fighters seizing oil fields and revenue, U.S. Defense Secretary Mark Esper said on Friday.

Iranian Foreign Minister Mohammad Javad Zarif addressed the issue at a news conference in Geneva on Tuesday evening after meeting with his counterparts, Russia's Lavrov and Turkey's Mevlut Cavusoglu.

"Well it seems that the United States is staying to protect the oil. And at least President Trump is honest to say what the United States intends to do," Zarif said to laughter.

"Iran and Russia are there on the invitation of the Syrian government, and we intend to stay there as long as the Syrian government and Syrian people want us to be there," he said.

Lavrov said that the return of U.S. forces to Syria, after their transfer to Iraq, was "under the pretext of protecting oil deposits from the Islamic State (ISIL)."

"The essence is that any illegal exploitation of natural resources of a sovereign state without its consent is illegal and that is the view that we share," he said.

"Our U.S. colleagues are aware of our position and we will defend that position," Lavrov said.