Heavy fighting around Aleppo in northern Syria, as Syrian government forces seek to encircle rebels, could leave tens of thousands of people cut off from all humanitarian supplies, a medical charity warned Thursday.

The push by government forces and its allies has so far failed to close off the city’s opposition-held eastern sector, with rebels counter-attacking successfully, but there is no sign as yet the offensive is over.

“In the last few days things have gotten worse. We are quite concerned by the increase in the combat,” said Teresa Sancristoval, the emergency desk manager with charity Doctors Without Borders (MSF).

Sancristoval told dpa that as many as 200,000 people could end up isolated, with no access to food, medical supplies or ability to get treatment for the wounded. Water supplies are already limited and there are shortages of soap and other hygiene materials.

“Enclaves are always traumatic for the population. This could be one of the most dangerous things for the people, to lack food, soap, medicine and petrol. It would really make the population weaker,” she said.

MSF has had to stop its operations in Aleppo, where it operates a full hospital that is able to carry out surgeries as well as a smaller medical centre.

The group’s access to the city is often hindered by security concerns, including bombardments.

Analysts say that losing Aleppo would be a critical blow to mainstream rebels, who are caught between regime advances and the Islamic State group.

The fighting comes as UN Syria envoy Staffan De Mistura announced that Damascus was willing to suspend its aerial bombardment of Aleppo – the country’s second city before the war – for a period of six weeks.

The opposition said it doubted the seriousness of the regime’s offer, saying President Bashar al-Assad lacked credibility.

The United States said Wednesday it had finished a screening process and identified 1,200 Syrian rebels who would be eligible to receive training to fight the Islamic State, which holds a swathe of eastern and north-eastern Syria.

The Syrian opposition has long complained the US has been slow to provide training to the rebels.

But former US ambassador to Syria Robert Ford, a long-time advocate for arming the rebels this week reversed his position, saying moderate forces are disorganized, and are often allied with Syrian al-Qaeda affiliate the al-Nusra Front.

Analysts say al-Nusra has played a key role in the few rebel successes in recent months, including a much-touted offensive that has driven regime forces out of areas in southern Syria. SAPA