WASHINGTON -- The United States' top military officer said on May 19 that a U.S. attack on a convoy of pro-Syrian government forces was strictly a "force protection measure" aimed at protecting U.S. forces and allies on the ground in Syria.
The May 18 strike was the first such attack by the United States on forces loyal to Syrian President Bashar al-Assad since last month's cruise missile attacks on a Syrian air base.
The convoy attack was condemned by Syria and by its ally Russia.
"It is illegitimate, it is unlawful and the latest gross violation of the sovereignty of the Syrian Arab Republic," Russian Foreign Minister Sergei Lavrov said.
Kremlin spokesman Dmitry Peskov said Russian Security Council members and President Vladimir Putin were discussing the U.S. strike in Syria, which he called "illegitimate."
The Pentagon said in a statement on May 18 that the Syrian-allied forces were threatening U.S. troops and allied forces operating near the town of Al-Tanf, on Syria's border with Jordan. The forces were hit with air strikes after they continued in their movements despite intervention by Russia and warning shots.
U.S. forces have been operating in the Al-Tanf area for months training partner forces to battle Islamic State (IS) fighters.
At a Defense Department briefing May 19, General Joseph Dunford, chairman of the Joint Chiefs of Staff, called the strikes a "force protection measure" and would happen again only if U.S. forces were threatened.
U.S. Defense Secretary Jim Mattis, meanwhile, said the Syrian-allied forces appeared to have been directed by Iran -- Syria's closest ally in the Middle East -- but it was unclear whether there were actually Iranian units among the forces that were struck.
Syrian activists said the U.S. attack killed at least eight fighters and that they appeared to be "non-Syrian."
An unnamed Syrian military official told state TV that the attack killed several soldiers and caused material damage.
The United States has led a coalition bombing IS fighters in Syria since 2014, but in the past has avoided engaging directly with Syria or Iranian-backed forces battling rebel forces there.
With both Russian and U.S. jets flying sorties in Syrian airspace, Moscow and Washington have set up a line of communication to avoid mid-air confrontations, something that has been successful to date.
Elsewhere in Syria on May 18, Islamic State militants attacked several government-held villages in central Syria, capturing at least one of them.
The attack left 52 people dead, including more than two dozen women and children, some of whom were beheaded, as well as Syrian troops, the monitoring group said.
The observatory said it was the most violent IS attack so far this year.