Russia on Friday accused the United States of committing a "war crime" in Syria, saying its military had denied Syrian refugees access to humanitarian aid.
The claim, denied in Washington, was made by the Russian defense ministry's Syria reconciliation center, which cited a "most severe humanitarian situation" in the Al-Tanf region.
A garrison there is being used by US and other international troops to train anti-ISIS fighters.
"The actions of the US military and the so-called international coalition in Al-Tanf are a gross violation of international humanitarian law and could qualify as a war crime," said a statement quoted by Russian news agencies.
It also said tens of thousands of Syrian refugees from the Al-Rukban refugee camp on the Jordanian border are deprived of humanitarian aid because of the US, which "illegally placed their military base there and forbids approach within 55 kilometers under the threat of death."
Washington strongly refuted Russia's allegations, insisting it has "provided extensive financial and operational support to UN agencies operating in Rukban" throughout the conflict.
"The US has advised the Russians that we are prepared to help facilitate the movement of humanitarian aid, but the Syrian regime has prevented those efforts," Department of Defense spokesman Eric Pahon told AFP.
"The Russians agreed to a 55km deconfliction zone around the Al-Tanf garrison to avoid accidental conflict between our forces.
Understandably, Syrian civilians have fled into this zone to flee bombing of non-military targets -- their homes -- outside the 55km line," he said.
Pahon urged Russia to focus its efforts on "pressuring the Syrian regime to allow UN humanitarian aid convoys to reach the camp and provide needed aid."
Earlier this month Moscow accused the US of supporting ISIS militants and enabling them to mount counter-offensive attacks in eastern Syria from near the Al-Tanf garrison.
Al-Tanf, on a highway connecting Damascus with Baghdad, has been repeatedly menaced by a surge of Iran-backed troops loyal to Syrian President Bashar al-Assad.
Washington says the garrison is used by US and British special forces to train rebel Syrian groups fighting ISIS.
Russia has been conducting a bombing campaign in Syria since 2015, when it stepped in to support the Assad regime and tipped the conflict in his favor.