The United States fired cruise missiles from two destroyers in the Mediterranean at a base from which President Donald Trump said a deadly chemical weapons attack had been launched on Tuesday. It was the first direct U.S. assault on the government of Bashar al-Assad in six years of civil war.
"Any military operation by a state on the territory of another without the consent of the other amounts to an international armed conflict," ICRC spokeswoman Iolanda Jaquemet told Reuters in response to a query.
"So according to available information - the U.S. attack on Syrian military infrastructure - the situation amounts to an international armed conflict," she said, an assessment confirmed by Andrew Clapham, an international law professor at the Graduates Institute in Geneva.
Washington says Syrian government forces carried out a poison gas attack on the town of Khan Sheikhoun in rebel-held Idlib province this week that killed at least 70 people, mainly civilians including children.
Syrias army command has denied responsibility for the attack. Russias defence ministry said the gas contamination was the result of a leak from a rebel chemical weapons depot after it was hit by Syrian government air strikes.
Previous air strikes on Syrian territory by a U.S.-led coalition have been against only the militant group ISIS, which is also the enemy of the Syrian government.
Russia has carried out air strikes in tandem with its ally Assad since September 2015, while Iranian militias are also fighting alongside the Syrian presidents forces against a wide range of rebel groups and Islamist militants.
Turkey is also involved, having launched an incursion into northern Syria more than six months ago to push ISIS insurgents away from its border and stop the advance of Kurdish forces. Turkey said last week that the operation had been completed successfully.
Under international humanitarian law, civilians must be spared and medical facilities protected, regardless of whether a conflict is internal or international.
Warring parties must observe key principles of precaution and proportionality and distinguish between combatants and civilians, and between military and civilian infrastructure.
Those principles have been violated by various sides in Syria, Western leaders and U.N. war crimes investigators say.
In an international armed conflict, rules also govern treatment of captured combatants who are deemed prisoners of war.
The designation creates a legal obligation to allow the ICRC to check on prisoners treatment and conditions of detention, and ensures their protection until the end of the conflict, Jaquemet said.
"There is a legal right to visit PoWs (prisoners of war) in the hands of the enemy," she said.
The ICRC is raising the U.S. attack with U.S. authorities as part of its ongoing confidential dialogue with parties to the conflict, Jaquemet added, without elaborating.