BEIRUT - A US-led coalition air strike on the eastern Syrian town of Mayadeen early Friday killed at least 80 relatives of Islamic State group fighters, a monitoring group told AFP.

"The toll includes 33 children. They were families seeking refuge in the town's municipal building," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights. "This is the highest toll for relatives of IS members in Syria," Abdel Rahman told AFP. The latest strike came as the United Nations urged all nations bombing jihadist targets in Syria to better distinguish between civilian and military targets.

UN human rights chief Zeid Ra'ad Al Hussein said "all states" whose air forces are active in the anti-IS missions needed "to take much greater care to distinguish between legitimate military targets and civilians."

The Britain-based Observatory gathered information from civilian and medical sources on the ground in IS-held Mayadeen, which was facing its third day of fierce bombing.

According to the Observatory, 37 civilians were killed in coalition raids on the town on Thursday night, including 13 children, and another 15 had been killed in coalition strikes on Wednesday. The 68-member coalition began bombing IS targets in Iraq in the summer of 2014, and expanded their operations to Syria on September 23 of that year.

This week, the Observatory reported the highest monthly civilian death toll for the coalition's operations in Syria.

Between April 23 and May 23 of this year, coalition strikes killed a total of 225 civilians in Syria, including dozens of children.  The US-led alliance is backing twin ground offensives against IS's last main bastion cities: Raqa in northern Syria and Mosul in neighbouring Iraq.  On Thursday, a Pentagon investigation concluded that at least 105 civilians died in an anti-jihadist air strike on an IS weapons cache in Mosul in March.

Prior to the new revelation, the US military had said coalition air strikes in Iraq and Syria had "unintentionally" killed a total of 352 civilians since 2014.

Syria army takes control of

Damascus-Palmyra road

The Syrian army is in full control of the highway from Damascus to ancient Palmyra for the first time since 2014 after driving out jihadists, a monitor said on Friday.

Since troops recaptured Palmyra from the Islamic State group in March, the UNESCO World Heritage site had been accessible via Syria's third city Homs, about 150 kilometres (95 miles) to the west.  But after major advances on Thursday night, the army now controls the direct road from the capital to Palmyra, the Syrian Observatory for Human Rights said.

With the support of Russian air strikes, regime fighters "pushed IS fighters out of desert territory amounting to more than 1,000 square kilometres (390 square miles)," Observatory head Rami Abdel Rahman told AFP.

"The jihadists were withdrawing because of how intense the Russian air strikes were," Abdel Rahman said.

Palmyra's temples, colonnaded alleys and elaborately decorated tombs - some of the best preserved classical monuments in the Middle East - attracted more than 150,000 tourists a year before civil war broke out in Syria in 2011.

IS fighters first overran it in May 2015, and government troops recaptured it 10 months later.

The jihadist group retook it in late 2016, but Russian-backed Syrian forces wrested back control in March.

A decades-old ally of Damascus, Moscow has been carrying out air strikes in support of President Bashar al-Assad's troops since September 2015.

The Syrian army appears to be conducting a multi-pronged drive towards the oil-rich territory near the country's eastern borders with Iraq and Jordan.

But parts of the frontier are controlled by rebel groups backed by the US-led coalition fighting IS.

Earlier this month, coalition warplanes struck a convoy of pro-government forces headed towards a remote coalition garrison near the Syrian-Jordanian border.

The desert, known as the "Badiya," extends over some 90,000 square kilometres (35,000 square miles) and makes up about half of Syria's territory.