Rebels including al-Qaeda militants stormed a complex in northwestern Syria on Sunday where some 250 government loyalists have been trapped for two weeks, a monitoring group said.

The Syrian Observatory for Human Rights said rebels entered the hospital complex in the town of Jisr al-Shughur, the rest of which they captured a fortnight ago.

"The fighters this morning stormed the hospital complex on the southwestern edge of Jisr al-Shughur," Observatory director Rami Abdel Rahman told AFP.

"They entered one of the buildings and are engaged in heavy fighting with soldiers inside the complex."

He said the assault began with a car bombing carried out by a member of al-Qaeda's Syrian affiliate, al-Nusra Front.

It is the first time that the rebels have managed to penetrate the complex.

Among the 250 people holed up inside are around 150 government troops, apparently including high-ranking officers, as well as their family members and some civil servants, according to Abdel Rahman.

On social media, there have been an unverifiable suggestions that some of the officers inside the hospital are Iranian and Russian.

#Syria #Idlib Rumors sprout about presence of high rank officers from #Russia & #Iran inside National Hospital of Jisr al-Shughour — C4H10FO2P (@markito0171) May 9, 2015

Government troops and militia have been battling to reach the hospital to relieve the siege.

They began their counteroffensive on Wednesday, as President Bashar al-Assad pledged those in the hospital would be rescued soon, and are now around two km away, according to the Observatory.

They have been backed by airstrikes against the rebels laying siege to the hospital, which state media said had killed "dozens of terrorists."

It remains unclear how much food and ammunition those holed up inside the hospital complex have left.

Turning of tides?

On 25 April, rebels seized the city of Jisr al-Shughur in an offensive that some analysts described as potentially game changing, challenging a recent sense in some quarters that Syrian President Bashar al-Assad was on the verge of reasserting control over the country.

One major change in the offensive was a new level of organisation and cooperation among leading opposition groups previously at one another's throats.

"So far, Assad has been winning mainly due to the fragmentation of the other side and the lack of skill and experience on the other side,” Randa Slim, director of the Track II Dialogues initiative at the US-based Middle East Institute, told MEE last week.

“Now that these two factors on the opposition side have been addressed in some way, I think we are seeing the difference in the playing field," she said.

The capture of the city extended the various groups' gains in Idlib province, where they have also captured the provincial capital and a military base in recent weeks.

More than 220,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011 that spiralled into civil war after a crackdown by security forces.