Beirut (AFP) - Lebanese Shiite militant group Hezbollah on Saturday blamed Sunni extremists for killing its top military commander in Syria and vowed to keep fighting to defend President Bashar al-Assad's regime.

The Islamist State (IS) jihadist group, meanwhile, briefly overran a government-controlled hospital in the eastern city of Deir Ezzor, triggering a battle with pro-regime forces in which 59 fighters were killed, a monitor said.

Hezbollah has deployed thousands of fighters in Syria where Mustafa Badreddine had led its intervention in support of Assad's forces, which are also backed by Russia and Iran.

Badreddine, who was on a US terror sanctions blacklist and wanted by Israel, was killed in an explosion on Thursday night near Damascus international airport.

Hezbollah announced his death on Friday but without immediately apportioning blame, breaking with its usual pattern of accusing arch-foe Israel of responsibility.

On Saturday, it said a probe had concluded that Sunni Islamist radicals known as "takfiris", who consider Shiites to be heretics, had killed Badreddine.

"An investigation has shown that the blast that targeted one of our positions near the Damascus international airport that led to the martyrdom of the brother commander Mustafa Badreddine was caused by artillery bombardment carried out by takfiri groups present in that region," a Hezbollah statement said.

It did not name any specific group, and there has been no claim of responsibility.

Hezbollah has been battling opponents of Assad's regime including Sunni extremists from IS and Al-Nusra Front, Al-Qaeda's Syria affiliate.

- Medics held hostage -

A Syrian security source has told AFP that Badreddine was in a warehouse near the airport when it was rocked by a blast on Thursday night.

No aircraft was heard before the explosion, the source said.

The head of the Britain-based Syrian Observatory for Human Rights, Rami Abdel Rahman, said no artillery fire had been heard in the area either in the past three days.

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In Deir Ezzor, IS attacked Al-Assad hospital on Saturday as it pressed an advance aimed at controlling all of the oil-rich city and its vital airbase, the Observatory said.

The attack sparked clashes with regime forces providing security for the hospital in which 35 soldiers and pro-regime militiamen and 24 IS fighters were killed, Abdel Rahman said.

The monitoring group said regime forces recaptured the hospital after losing control for several hours during which IS took medical staff hostage.

There was no immediate word on their fate.

The jihadist group controls about 60 percent of Deir Ezzor, including the centre and the north of the city.

- Kerry peace push -

In the northern city of Aleppo, where a ceasefire expired on Wednesday at midnight, the Observatory reported seven civilians killed in 48 hours of rebel shelling of government-held western sectors.

State television said a reporter and three other people were wounded by rebel rocket fire on state radio and television offices in Aleppo.

The Observatory also reported heavy fighting on the edge of rebel-held Daraya near Damascus, besieged by government forces since 2012 and where a Red Cross operation to deliver humanitarian aid this week was thwarted.

Syria's conflict has killed more than 270,000 people and displaced millions since it started with the repression of anti-government protests in 2011.

On the political front, US Secretary of State John Kerry arrived in Saudi Arabia on Saturday to consult with his Arab ally ahead of international talks in Vienna next week aimed at salvaging teetering peace efforts.

In Hezbollah's statement on Saturday, a day after thousands attended Badreddine's funeral in Beirut, Hezbollah vowed no let-up in its war against those it describes as "criminal gangs" in Syria.

Badreddine was a key suspect in the 2005 assassination in Beirut of Lebanese ex-premier Rafiq Hariri and was one of Israel's "most wanted".

His predecessor, cousin and brother-in-law Imad Mughniyeh, was killed in Damascus in a 2008 bombing that Hezbollah blamed on Israel.

Hezbollah has also accused Israel of killing another of its prominent figures, Samir Kantar, in an air strike last December near Damascus.

Expert Waddah Charara says Hezbollah has sent between 5,000 and 6,000 combatants to Syria.

Between 1,000 and 2,000 of its fighters have been killed in combat there, other experts say.