BEIRUT — Thousands more people have fled violence in northwest Syria, the United Nations and a medical agency said on Thursday, as an army assault on the last big rebel enclave met a counter-attack.

President Bashar Al-Assad launched his offensive at the end of April in Idlib and parts of adjacent provinces with an intense bombardment, saying insurgents had broken a truce.

This week, rebels rolled back some government advances on the main battlefront, retaking the town of Kafr Nabouda.

Government forces are buttressed by Russian air power, while the main jihadist group that dominates Idlib has been reinforced by Turkey-backed rebels.

Eight years into the civil war, Assad has retaken most of Syria and rebels still fighting him are squeezed into the northwest. Turkey-backed groups hold a strip of territory on the border, and Kurdish-led fighters hold the northeast.

This week's fighting brought a big increase in air strikes, with bombs falling on towns and villages across the southern part of the enclave, said a British-based war monitoring group, the Syrian Observatory for Human Rights.

Some 600 air strikes hit the rebel enclave on Thursday, the Observatory said, killing six civilians.

More than 200,000 people have now fled the violence since the end of April, the United Nations said, and are in urgent need of food and protection.

The Union of Medical Care and Relief Organizations (UOSSM), which provides assistance to health facilities, said the number of displaced this month had spiked to more than 300,000.

Most of the displaced have sought refuge along the border with Turkey, the UOSSM said, with camps springing up in the shadow of the frontier wall.

However, 44,000 people have moved to the regional capital Idlib and another 50,000 have gone to Maarat Al-Numan, another large town where the Observatory said an air strike on a marketplace killed 12 people on Tuesday night.

The bombardment has used both conventional air strikes by war planes and "barrel bombs" — improvised explosives dropped by helicopter - according to the Observatory and rescue services.

Since the end of April, there have been 20 attacks on healthcare facilities and one on an ambulance, the United Nations said, putting 19 facilities that serve at least 200,000 people out of action. Some were hit more than once, it said.

The Observatory said 669 people have been killed since the end of April, 209 of them civilians. The UOSSM said 229 civilians had been killed in that period.

Rebels fighting on the mountainous western edge of the enclave said on Sunday that the army had shelled them with poison gas, leading some to suffer choking symptoms.

The US State Department warned it would respond "quickly and appropriately" if that was proven.

However, US Syria envoy James Jeffrey said on Wednesday that Washington did not have confirmation that poison had been used. Rebels said they had not documented the attack because they were under bombardment when it occurred.

Syria's government denies using chemical weapons. — Reuters