The United States fired cruise missiles at a base from which President Donald Trump said a deadly chemical weapons attack had been launched on Tuesday, 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 Geneva 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."
Previous airstrikes on Syrian territory by a U.S.-led coalition have been against only the militant group Islamic State, which is also the enemy of the Syrian government.
Russia has carried out airstrikes in tandem with its ally Syria since Sept. 2015, while Iranian militias are also fighting alongside the troops of Syrian President Bashar al-Assad.
ICRC officials were raising the U.S. attack with U.S. authorities as part of its ongoing confidential dialogue with parties to the conflict, Jaquemet said, declining to give details.
The ICRC, guardian of the Geneva Conventions setting down the rules of war, declared Syrian internal armed conflict — or civil war, in layman's terms 3 — in July 2012.
Under international humanitarian law, whether a conflict is internal or international, civilians must be spared and medical facilities protected. Warring parties must observe the key principles of precaution and proportionality and distinguish between combatants and civilians.