WASHINGTON (Reuters) - The United States will counter any attempt to wrest control of Syria's oil fields from US-backed armed groups, the Pentagon said on Wednesday.

The ministry said on Monday that US forces would prevent any party from taking control of these fields, "whether the opponent is Islamic State or forces supported by Russia or Syria."

More than once, the United States has stressed that it will keep troops to protect Syria's oil fields, despite the president's decision to withdraw his troops from there.

Last week, the US military announced that it would strengthen its position in Syria with reinforcements, including mechanical forces, to prevent the seizure of oil fields by remnants of the Islamic State or others.

On Monday, Defense Secretary Mark Esper said, "US forces will remain in this strategic area to prevent ISIL from reaching these vital resources, and we will respond with overwhelming force to any group that threatens the safety of our troops there."

Asked by reporters whether the US military's mission included preventing any Russian or Syrian government forces from reaching the oil fields, Esber said: "The short answer is yes, it already exists."

"We want to make sure that the SDF can access these resources in order to guard the prisons, arm their forces and help us in the task of defeating IS," he said.

Two days ago, the United States began sending military reinforcements to Syria's oil-rich east.

A US military convoy was seen heading south of the Syrian city of Qamishli towards Deir Ezzor, where most of the oil fields are located.

Washington has begun strengthening its positions in Deir Ezzor province, where Syria's main oil fields are located, in coordination with the so-called Syrian Democratic Forces (SDF), which is dominated by the Kurdish People's Protection Units (YPG).

The correspondent told Agence France-Presse that a US convoy of about 13 military vehicles entered Syria from Iraq, and went to the neighboring province of Hasaka.

He added that the convoy crossed the checkpoints of the Syrian regime, and crossed the city of Qamishli, a predominantly Kurdish (northeast).

Earlier, the Russian Defense Ministry described the US control of the oil fields in eastern Syria as the conduct of bandits and the practice of "banditry" on a global level.