Assad-backers Russia accuse US of blocking aid in Syria
Moscow's accusations centre on a military base in the al-Tanf region near the Jordan border, where the US coalition is training rebel forces to fight Islamic State.
It says its "illegal" presence there has prevented Syrian refugees in the Rukban refugee camp on the Jordanian border from reaching humanitarian aid, because the US "forbids approach within 55 kilometres (35 miles) under the threat of death".
The US military operates a "deconfliction" zone and has previously warned Assad-supporting Iran-backed militias, which it views as a threat, not to interfere with anti-IS operations at the training facility.
The Rukban camp has also frequently been targeted by IS car bombs.
"The most severe humanitarian situation remains in the al-Tanf region," the Russian defence ministry's Syria reconciliation centre said.
"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," it added in a statement quoted by Russian news agencies.
Russia is backing the Syrian regime in the six-year civil war which has cost hundreds of thousands of civilian lives, besieged cities for months without adequate food and medical supplies, and caused devastation to homes, hospitals and schools.
The UN and other humanitarian agencies including Doctors Without Borders (MSF) have frequently accused the Syrian regime and its allies, predominantly Russia, of blocking aid from reaching beseiged areas.
Last month the International Committee of the Red Cross warned that humanitarian needs in beseiged Eastern Ghouta were "huge" and that the "situation is getting worse".
Russia has also systematically vetoed UN Security Council resolutions calling for an immediate halt in air strikes.
Russia's accusations against the US come a day after it demanded the UN Security Council shelve a report blaming the Syrian government for a sarin gas attack. Moscow called for a new probe into the use of the deadly nerve agent, according to a draft resolution obtained by AFP.
Russia has questioned the work of the panel because the experts did not go to Khan Sheikhun - the site of the attack - and worked from samples Moscow maintains may have been tampered with by Western intelligence.
The White House issued a strong statement denouncing Moscow's attempts to undermine the work of the panel, which Russia helped establish with the United States in 2015.
"Russia's attempts to undermine and eliminate the JIM [Joint Investigative Mechanism] show a callous disregard for the suffering and loss of life caused by the use of chemical weapons and an utter lack of respect for international norms," White House spokeswoman Sarah Sanders said.
Previous reports by the JIM have found that Syrian government forces were responsible for chlorine attacks on three villages in 2014 and 2015, and that the Islamic State group used mustard gas in 2015.
The Syrian conflict began when the Baath regime, in power since 1963 and led by Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.
Agencies contributed to this report.