Syria and its ally Russian Federation have condemned air strikes by the US-led coalition on pro-Syrian government forces near the Jordan border.
The attack was the first close confrontation between USA forces and fighters backing President Bashar Assad.
A Syrian military official confirmed that a USA air strike in the area of al-Tanf this week killed a number of people, according state-run news.
While contact with ISIS units is to be expected considering the mission, this is the first instance of American air power being used against pro-Assad forces to support American troops and their rebel allies.More news: Roadside bomb kills 11; attacks kill 10 Afghan policeman
The official said an earlier communication agreement between the United States and Russian Federation in Syria had included ways to avoid mishaps between ground forces, such as establishing a 55-kilometer border around known areas of operation.
Syrian government forces secure a road for a military convoy in the desert area near the border with Jordan and Iraq, May 10, 2017.
The strike was a defensive move to protect allies, U.S. officials said.
US Defense Secretary James Mattis told reporters on Friday that US President Donald Trump has authorized commanders in the field to conduct some strikes without direct Pentagon approval, a shift from the previous administration.
Russian Federation is a key ally of Syrian dictator Bashar al Assad's regime, which called the US-led action a "brazen attack" and insisted it would "not be intimidated" by the assault.More news: China building biggest infrastructure project in history
The Syrian military official said attempts to justify the attack by claiming the toops did not respond to a warning not to advance are "categorically rejected". He said: "We are not increasing our role in the Syrian civil war".
A convoy of 20 pro-regime vehicles were headed toward At Tanf Wednesday night, searching vehicles and buildings along the way, according to a U.S. official.
Western intelligence sources told Reuters that U.S. and British special forces are expanding the Tanf base to oust Islamic State militants from the eastern province of Deir Zor, that straddles Iraq, from the south east of Syria.
The April strikes were ordered in retaliation for a chemical weapons attack that Washington blamed on Damascus, and were described as a one-off measure to deter any future chemical weapons use.
"We welcome robust action against the foreign forces who have turned Syria into a killing field", he said.More news: Sunderland goalkeeper can succeed De Gea says Manchester United legend
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