Desperate Syrian refugees fleeing heightened violence in the north-west of Syria are being shot at and beaten at the Turkish border as they try to flee, Human Rights Watch says.

The human rights group said refugees who had succeeded in crossing to Turkey using smuggling routes told of Turkish border guards shooting at them during their crossing, while others reported asylum seekers being detained and denied medical assistance.

"Syrians fleeing to the Turkish border seeking safety and asylum are being forced back with bullets and abuse," said Lama Fakih, deputy Middle East director at Human Rights Watch.

"As fighting in Idlib and Afrin displaces thousands more, the number of Syrians trapped along the border willing to risk their lives to reach Turkey is only likely to increase."

In interviews Human Rights Watch conducted, witnesses described children being among those shot as they tried to cross, while others spoke of being hit by Turkish border guards and forced back after being caught.

One witness told HRW that a woman had given birth while attempting to cross the border but that Turkish border guards sent her and the child back to Syria without providing medical assistance.

The Turkish border is closed to Syrian refugees except for critical medical cases.

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The reports of the conditions at the Turkish border come as violence increases in the last rebel-held stronghold in north-west Syria, as Syrian regime forces advance into the province.

The Britain-based Syrian Observatory for Human Rights said 40 civilians has been killed in the past four days as bombing escalated.

Warplanes and helicopters struck in Idlib, Hama and Aleppo provinces, including in the towns of Saraqeb and Kafr Nabouda, said the war monitor.

Rescue agencies reported at least seven civilians, including two children and a woman, were killed after Syrian government warplanes targeted the vehicles of families fleeing fighting along the Aleppo-Damascus international road on Friday.

Meanwhile, three civilians were reported killed, including a child, after heavy Syrian army shelling and air raids on Douma, a rebel-held town in Eastern Ghouta near the capital Damascus.

International concern has been rising over the fate of 400,000 people living in besieged, rebel-held eastern Ghouta as acute food and medicine shortages have contributed to what the United Nations has called the worst malnutrition of the war.