Russian officials have accused the United States of using banned phosphorous bombs in an attack on Syria on Saturday. It marks the latest is a series of escalations between the United States and Russia bringing tensions to a Cold War high.

“In Deir ez-Zor province on September 8, 2018, two F-15 aircraft of the US Air Force carried out strikes on the settlement of Hadjin with the use of phosphorus incendiary munitions. As a result of the strikes, major fires were observed. Information on victims and injured is being clarified,” Major-General Vladimir Savchenko of the Russian Center for Syrian Reconciliation said in a statement.

White Phosphorous munitions were banned as part of the 1949 Geneva Convention.

The Russian military has not said what casualties, if any, were reported.

The Pentagon has denied the charges. “None of the military units in the area are even equipped with white phosphorous munitions of any kind,” it said in a statement.

But it is not the first time that the United States has been accused of using white phosphorous in its war against the Islamic State in Syria. Human rights groups have charged the United States coalition of using the munitions throughout its war with ISIS, Reuters reported.

The tension between the two powerful factions has reached the boiling point in recent days. President Donald Trump and his administrations are said to be weighing military options if the Syrian government uses chemical weapons, as expected, against the last rebel stronghold of Idlib.

And Russia has levied its own threats against the United States military. Officials from the Russian military have threatened American troops directly if they are in the way of their effort to helps Syrian President Bashar al-Assad rid his nation of its rebels.

US Central Command’s Lieutenant Colonel Early Brown Russia informed him “that they intended to enter the At Tanf deconfliction zone to pursue terrorists.”

The United States responded by deploying 100 US Marines to the At Tanf outpost and made a declaration to Russia to back off.

“The US does not require any assistance in our efforts to destroy ISIS in the At Tanf deconflicition zone and we advised the Russians to remain clear,” Gen. Brown said in a statement to the Los Angeles Times.

“The United States does not seek to fight the government of Syria or any groups that may be providing it support. However, if attacked, the United States will not hesitate to use necessary and proportionate force to defend US, coalition or partner forces,” a United States defense official told CNN.

“We have absolutely advised them to stay out of At Tanf … We are postured to respond,” another official said.

United States troops are set to conduct a military exercise at Tanf with live ammunition.