The US-led coalition has killed 18 'friendly fighters' in Syria after mistaking them for ISIS fanatics, it has emerged.

An airstrike accidentally targeted a Syrian Democratic Forces (SDF) unit which was battling the terror group south of Tabqa, which is near the strategically important Euphrates River.

It is not clear which air force was behind the April 11 strike, which hit advancing allied fighters not far from the Tabqa dam and military airport close to the ISIS stronghold of Raqqa.

It is understood the victims were killed after giving the wrong coordinates to the US.

An airstrike accidentally targeted a Syrian Democratic Forces (SDF) unit which was battling the terror group south of Tabqa, in the province of Deir al-Zor (file picture)

The US-led coalition said in a statement: 'The strike was requested by the partnered forces, who had identified the target location as an ISIS fighting position.

'The target location was actually a forward Syrian Democratic Forces fighting position.

'The coalition's deepest condolences go out to the members of the SDF and their families.

'The coalition is assessing the cause of the incident and will implement appropriate safeguards to prevent similar incidents in the future.'

WHO ARE THE SDF? The Syrian Defence Forces (SDF) is a predominantly-Kurdish force from the People's Protection Units, or YPG, but also includes ethnic Assyrians, Turkmen, Armenians and Arabs. The US-led coalition is supporting the SDF with arms, air strikes, training and advice as they fight ISIS. Advertisement

Officials were assessing the cause and would 'implement appropriate safeguards to prevent similar incidents in the future,' the statement added.

The strike occurred on April 11, just days after Donald Trump ordered a wave of missile strikes on one of dictator Bashar al-Assad's airbases.

SDF troops backed by coalition air power and other military assistance have been fighting for control of Tabqa since March.

The town is considered an important waypoint ahead of the main offensive for Raqqa, ISIS' last bastion in Syria.

In its own statement, the SDF described the incident as a 'painful accident' resulting from a 'mistake'.

'The accident caused a number of deaths and injuries,' the statement said.

'The general command of the Syrian Democratic Forces is coordinating with international coalition forces to investigate the incident and uncover the reasons and conditions that led to this accident.'

The US-led coalition has killed 18 'friendly fighters' in Syria after mistaking them for ISIS fanatics, it has emerged. It is not clear which airforce carried out the strike (file picture)

Washington has blamed a chemical weapon attack last week on President Bashar al-Assad's regime, and launched cruise missiles at a Syrian air base last week

The friendly fire comes as the Syrian army claimed a separate US-led air strike hit poison gas supplies belonging to jihadists, releasing a toxic substance that killed 'hundreds including many civilians'.

The bombing in the eastern Deir al-Zor province late on April 12 proved ISIS and al Qaeda-linked militants 'possess chemical weapons', a statement by the army flashed on Thursday by Syrian state TV said.

The report could not immediately be independently verified.

The U.S.-led coalition against Islamic State on Thursday denied the Syrian army report.

U.S. Air Force Colonel John Dorrian, a spokesman for the coalition, said it had carried out no air strikes in the area of Deir al-Zor at the time.

'The Syrian claim is incorrect and likely intentional misinformation,' Dorrian said in an email to Reuters.

The Russian defence ministry said it had no information about people killed in an attack by international coalition forces in Deir al-Zor, according to RIA news agency. A ministry spokesman said Russian forces had sent drones to check the area.

Earlier, Russia's foreign minister said Moscow expects the UN's chemical weapons watchdog to conduct an extensive probe into last week's chemical attack in Idlib, Syria.

Sergey Lavrov said that inspectors from the Organisation for the Prohibition of Chemical Weapons should visit the affected areas to get a full and objective picture.

The US blamed the Syrian government for launching the attack, but Russia claimed that the victims were killed by toxic agents released from a rebel chemical arsenal hit by Syrian planes.