The Syrian Observatory for Human Rights said rebels had 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. They entered one of the buildings and are engaged in heavy fighting with soldiers inside the complex," Observatory director Rami Abdel Rahman told AFP today.
He said at least 39 opposition fighters and "dozens" of regime forces had been killed in fighting inside the hospital and also outside Jisr al-Shughur during the army advance.
It is the first time the rebels have managed to penetrate the complex.
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Among the 250 people holed up inside are around 150 regime troops, including "high-ranking officers", as well as family members and some civil servants, according to Abdel Rahman.
Rebels seized Jisr al-Shughur on April 25, extending their gains in Idlib province where they have also captured the provincial capital and a military base in recent weeks.
Government troops and militia have been battling to reach the hospital to relieve the siege.
They have been backed by air strikes against the rebels laying siege to the hospital, which state media said had killed "dozens of terrorists".
Late Sunday afternoon, Syrian state television said two local reporters covering the fighting in the area had been wounded by "terrorists targeting" the media reporting on the battle.
It remains unclear how much food and ammunition those holed up inside the hospital complex have left.