This is the moment US warplanes launched an airstrike on a convoy of pro-Assad fighters as they tried to advance towards an American-backed rebel base in Syria.

Footage shows men walking towards the scene of a blast near the southern town of Tanf with smoke rising from a military-style vehicle.

But seconds later another rocket slams in to the convoy sending a huge cloud of smoke into the sky.

The air raid targeted a militia loyal to Syrian dictator Bashar al-Assad near the border with Jordan, destroying at least one tank and a bulldozer.

Jets fired warning shots to stop the convoy of Syrian and Iranian-backed fighters from advancing - but it was ignored. The coalition said 'apparent' Russian attempts to stop the group, which had come to within 17 miles of the base, also failed.

Syria condemned the strike as a 'brazen attack' and said it would 'not be intimidated' after the surprise assault while Russia called America's actions 'unacceptable'.

Dramatic footage captures the moment US warplanes launched an airstrike on a convoy of pro-Assad fighters as they tried to advanc towards an American-backed rebel base in Syria

Video shows men walking towards the scene of a blast near the southern town of Tanf with smoke rising from a military-stlye vehicle

Seconds later another rocket slams in to the convoy sending a huge cloud of smoke into the sky

'Such actions that were carried out against the Syrian armed forces... this is completely unacceptable, this is a breach of Syrian sovereignty,' deputy foreign minister Gennady Gatilov was quoted as saying by state-run RIA Novosti in Geneva.

Last month, the US fired 59 missiles at a government air base in central Syria as punishment for a chemical attack blamed on Assad's forces that killed nearly 90 people.

This morning, Muzahem al Saloum, from the US-backed Maghawir al Thwra group said jets then struck after the militia advanced to a base used by the US and fighters supported by Washington.

'We notified the coalition that we were being attacked by the Syrian army and Iranians in this point, and the coalition came and destroyed the advancing convoy,' Saloum said.

The region around Tanf, where the borders of Jordan, Syria and Iraq meet, has been considered a de-conflicted zone, under an agreement between the US and Russia.

Speaking to reporters, Defense Secretary Jim Mattis said the US will defend its troops in case of 'aggressive' steps against them. He was asked if the airstrike increases the US role in the Syrian war.

'We are not increasing our role in the Syrian civil war, but we will defend our troops,' Mattis said. 'And that is a coalition element made up of more than just U.S. troops, and so we will defend ourselves (if) people take aggressive steps against us.'

The 'defensive' strike was also an apparent signal to Assad to keep his forces out of a zone where US-backed rebels are fighting ISIS.

US warplanes have launched an airstrike on a convoy of pro-Assad fighters seen advancing towards a US-backed rebel base in Tanf, Syria

A man checks the damage at the mountain resort town of Zabadani in the Damascus countryside in Syria on Thursday. A US airstrike struck pro-Syrian government forces for the first time, hitting a convoy in the desert near the border with Jordan, officials and activists said

'This action was taken after apparent Russian attempts to dissuade Syrian pro-regime movement south... were unsuccessful, a coalition aircraft show of force, and the firing of warning shots,' the U.S.-led coalitions aid.

It said coalition forces have been operating in the area 'for many months training and advising vetted partner forces' in the battle against ISIS.

A Syrian military sources told state media the strike had produced 'a number of martyrs and material damage.'

'This brazen attack by the so-called international coalition exposes the falseness of its claims to be fighting terrorism,' it added.

'The Syrian Arab Army is fighting terrorism on its territory, and no party has the right to determine the course of its operations,' the source said.

'The Syrian Arab Army will... not be intimidated by the attempts of the so-called coalition to stop it from performing its sacred duties.'

The US strike marks a new approach in what has become an intensely crowded and complicated war zone. Thursday's strike was the coalition's first on pro-Assad forces in the battlefield. The coalition had so far kept its military operations focused on ISIS militants and al-Qaeda-linked groups.

An increasingly visible US role in Syria has also raised the possibilities of friction with the various forces on the ground.

America is backing Syrian Kurdish forces who are also fighting ISIS to the country's east.

The strikes would be the first against fighters aligned with Syria's government since the United States waged cruise missile strikes on a Syrian air base in April. The above image shows a Tomahawk missile being fired from the USS Porter on April 7, 2017

US troops have sent patrols in the area to act as a buffer between Turkish troops and the Kurdish fighters.

Turkey views the US-backed Kurdish fighters as an extension of its own insurgent group.

In recent days, near the border with Jordan, another set of U.S-backed rebel fighters have been on a collision course with government troops in the area of Tanf.

The government launched a new offensive in recent days in the area, and activists say pro-government militiamen, mainly from Iran and the Lebanese Shiite militant Hezbollah group, have deployed there aiming to secure the main highway that runs from Damascus to Baghdad and beyond, to Tehran.

Tensions have been building as part of a race for control of territory stretching from the provincial capital of Deir el-Zour in northeastern Syria to the Iraq border.

The area gained attention as the battle for the Iraqi city of Mosul escalated in recent weeks. An estimated 10,000 ISIS fighters uprooted from Mosul are believed to be massing in the border area.

US officials said the American airstrike hit the pro-Syrian government forces as they were setting up fighting positions in a protected area near Tanf. They said a tank and a bulldozer were also hit.

One official said the pro-regime forces had entered a so-called 'de-confliction' zone without authorisation and were perceived as a threat to US-allied troops there.

The air raid targeted a militia loyal to Syrian dictator Bashar al-Assad near the southern town of Tanf near the border with Jordan, destroying at least one tank and a bulldozer. A huge Syrian National flag hangs out of a damage building at the mountain resort town of Zabadani in the Damascus countryside

A Syrian opposition media group, the Palmyra News Network, said the attack occurred at the Zarka juncture, about 17 miles from the border, destroyed a number of vehicles and caused casualties.

The Syrian Observatory for Human Rights also said the strike destroyed vehicles and killed eight militiamen. There was no immediate comment from the pro-government side.

In September 2016, the coalition erroneously struck at Syrian government troops in Deir el-Zour, killing over 90 soldiers. The US at the time said it was a mistake, as it was targeting ISIS positions.

Further to the north, ISIS militants on Thursday attacked several government-held villages in central Syria, capturing at least one and killing 52 people.

The dead included more than two dozen women and children, some of whom were beheaded, as well as Syrian troops, according to state media, medical officials and an opposition monitoring group.

The US-backed Syrian Kurdish-rebels are closing in on Raqqa, the de facto capital of ISIS in Syria.

That battle has already caused clashes between government forces and Syrian rebels and raised concern of pro-government militias making a bid for controlling the border with Iraq.

The ISIS attack in the central Hama province, meanwhile, targeted villages where most residents belong to the Ismaili branch of Shiite Islam, raising fears of massacres such as those ISIS carried out in other minority communities in Syria and Iraq.

Jets fired warning shots to stop a convoy of fighters loyal to Syrian dictator Bashar al-Assad (pictured)

The villages are located near the town of Salamiyeh and the highway that links the capital, Damascus, to the northern city of Aleppo, but state media said traffic was not affected.

Media reports and doctors in the area said some of the killed, which included women and children, were beheaded and others dismembered.

ISIS extremists are notorious for mutilating bodies of their adversaries, particularly members of other sects than Sunni Islam.

The militants stormed homes in the southern part of Aqareb al-Safi village before government forces pushed them back into the desert, the state news agency SANA reported.

The head of the National Hospital in Salamiyeh, Dr. Noufal Safar, said it received 52 bodies, including 11 women and 17 children.

Some of the bodies were badly mutilated, beheaded or had their limbs severed but 'most appear to have died as a result of gunfire,' Safar told The Associated Press by telephone.

Rami Razzouk, a coroner at the hospital who inspected the bodies, said those of children were brought in mostly dismembered while the men had died from shelling or heavy machine-gun fire. He said at least nine children were beaten on the head with heavy objects such as bricks or stones.

The Observatory also said that 52 people were killed in the fighting, with the dead including 15 civilians, 27 Syrian soldiers and 10 unidentified people.

Razzouk said 120 people were wounded; SANA said 40 were wounded.

The ISIS-linked Aamaq news agency said the militants captured villages of Aqareb al-Safi and Mabouja. It identified residents as members of Assad's Alawite sect, an off-shoot of Shiite Islam. The Sunni extremists view Shiites as apostates deserving of death.

ISIS has massacred thousands of Shiites and other opponents in Syria and Iraq, often boasting about the killings and circulating photos and videos of them online.

Aamaq claimed that 100 Syrian troops and pro-government gunmen were killed in the fighting.

'Dozens of people are missing but it is not clear if they were kidnapped' by IS, the Observatory's chief Rami Abdurrahman said.