Syrian rebels' success in seizing three military bases in less than a week has underscored the growing difficulty faced by Damascus in securing its outposts and stopping a rebel encroachment that has claimed large swaths of the east and north of the country.

Attacks on the bases, one north-east of Aleppo, a second at Mayedin in the far east and a third near Damascus, yielded a large number of weapons, which had been in desperately short supply, especially in positions across Syria's second city.

The impact of the new weapons seemed to have been felt immediately along northern frontlines, where Kurdish groups loyal to the Assad regime were on Friday engaged in their heaviest clashes yet with rebel forces and jihadists, near the border town of Ras al-Ain.

Up to 30 fighters on both sides had been killed by nightfall, with the battle expected to rage throughout the weekend. Rebels and Kurdish groups clashed in Aleppo earlier this month but had since struck a detente.

Each of the bases raided had been among the last regime strongholds in their respective parts of the country and had in effect become fortresses in hostile territory. Rebel fighters, using captured regime soldiers as labourers, were seen carrying away hundreds of crates of guns, medium-range weapons and ammunition.

All the while, the most formidable weapon in the regime armoury – air force jets – were present in the skies nearby. The regime's jet fleet of Russian-made MiGs has remained a lethal threat since they were deployed in mid-summer shortly after opposition groups stormed both the capital, Damascus, and Aleppo.

Their menace was emphasised yet again on Wednesday when a bomb dropped from a jet scored a direct hit on the main opposition-held al-Shifa hospital in the Shaar district of Aleppo, killing up to 40 people.

Witnesses to the attack told the Guardian of carnage inside the building, which is in the middle of a commercial and residential district close to the centre of the city. They said the hospital was no longer functional and that wounded civilians or rebel fighters must now be treated in makeshift clinics.

Among the dead in al-Shifa were trauma surgeons and specialists who had treated scores of casualties each day for the past four months.

"It is a disaster what has happened here," said a local vendor, Khaled Homsi, contacted by telephone. "Did the [Israelis] do this in Gaza? Never. This is a terrible war crime and it must be investigated by an international tribunal."

Syria's air force is in effect levelling the battle field below, both with its bombs and its constant presence, which is taking a psychological toll on rebel groups that can do little to nullify it. Though in possession of a handful of looted anti-aircraft missiles as well as a small number smuggled from Turkey, fighter jets remain the most dominant weapon in the Syrian civil war.

Turkey this week said it would ask Nato to provide it with batteries of Patriot missiles to reinforce its restive southern border, where sporadic cross-border shelling has continued for much of the past three months.

Nato has said it will consider the request from one of its member states, in a move that has angered its cold war foe and staunch Syrian ally, Russia. The move is seen by rebels as an attempt to keep Syrian jets away from border areas.

Since the sustained border flare-up that started in late September, Syrian jets have seldom been seen near the border. However, they were active earlier in the month above the border town of Ras al-Ain where Friday's clashes were taking place. Turkey did not respond at the time.