The US military has carried out an air raid against a convoy of Syrian pro-government fighters who, it said, posed a threat to US-backed forces in the country's south near the Jordan border.

A US official said on Thursday the attack near the town of al-Tanf destroyed at least one tank and a bulldozer, while another official said they followed warning shots by US aircraft meant to dissuade the fighters from advancing further.

"We are not increasing our role in the Syrian civil war, but we will defend our troops," James Mattis, the US defence secretary, said on Thursday when asked about the attack.

"And that is a coalition element made up of more than just US troops, and so we will defend ourselves [if] people take aggressive steps against us."

However, the al-Tanf attack raises questions about the ongoing feasibility for the US-led coalition to maintain its singular focus on the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.

Syria's civil war began in 2011 after mass protests against the rule of President Bashar al-Assad and has killed hundreds of thousands of people, driven half the country's population from their homes and dragged in world powers.

Reaction to US attack

The Syrian Observatory for Human Rights (SOHR) said eight non-Syrian fighters were killed in the US attack.

The SOHR statement appeared to confirm a statement by an official from a US-backed Syrian rebel group who told Reuters news agency that the convoy comprised Syrian and Iranian militia members.

According to Western intelligence and rebel commanders quoted by Reuters, the Syrian army is seeking to retake the strategic Damascus-Baghdad highway, which has been used in the past to smuggle Iranian weapons into Syria.

Al Jazeera's Charles Stratford, reporting from Gaziantep on the Turkey-Syria border, said the al-Tanf attack is a "worrying development".

"We are beginning to get some reaction from the Syrian pro-government media, according to which there were pro-government militia members in the convoy, which was made up of five T62 tanks," he said.

"Two of those tanks were destroyed and six people were killed and three wounded."

Reporting from Geneva, where the sixth round of talks is taking place, Al Jazeera's Mohammed Jamjoon, said: "A military source said the US-led coalition will not allow the Iranian militias to reach the border between Syria and Iraq in order to have an opportunity to connect with Iranian militias on the Iraqi side.

"The source said that the Americans will have to protect their position because the alternative would be tantamount to a withdrawal that would leave the front line to ISIL and Iranian militias."

Residents killed

Earlier on Thursday, Syrian state media and SOHR said ISIL fighters attacked villages near the only useable road that links the government-controlled cities of Aleppo and Homs, killing many residents.

SOHR said 52 people, civilians and fighters on both sides, were killed and dozens injured in the attacks.

It said at least 15 of the victims were civilians, including five children, and three of them died in execution-style killings.

State-run SANA news agency said ISIL killed 20 people in the village of Aqarib al-Safi, east of Hama city, before the army and allied militia repulsed the attack.

SOHR said ISIL, also known as ISIS, seized Aqarib al-Safi and parts of the nearby village of al-Mabouja as the clashes raged.

ISIL confirmed on social media that its fighters had captured Aqarib al-Safi.

'Tightening the noose'

Elsewhere in Syria, a US-backed alliance of Kurdish and Arab fighters has seized 350 square kilometres in Raqqa province over the past week, "tightening the noose" on ISIL in an advance to isolate its base of operations, a spokesman for a US-led coalition said on Wednesday.

The Syrian Democratic Forces (SDF) alliance has been encircling Raqqa, ISIL's self-proclaimed capital in Syria, since November.

Last week, its forces accomplished a major goal by capturing Tabqa, a previously ISIL-held town about 50km west of Raqqa, and a strategic dam nearby.

Al Jazeera's Stratford said "there are reports of heavy fighting in the countryside to the north and east of [Tabqa].

"But there are also growing concerns about what the UN describes as hundreds of thousands of people who are expected to flee the fighting as it intensifies in the days and weeks ahead."