A rebel coalition seized the Syrian government's largest remaining military base in northwest Idlib province on Tuesday, after an explosion and heavy clashes, a monitor said.

"All regime forces have withdrawn from Al-Mastumah, the largest regime base in Idlib, leaving it completely in the hands of opposition fighters," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP.

The rebel alliance that seized the camp calls itself the "Army of Conquest" and includes Al-Qaeda affiliate Al-Nusra Front and other Islamist factions.

"With the help of God, Al-Mastumah was completely liberated after Al-Nusra Front stormed it from the south," the group wrote on its official Twitter account.

The loss of Al-Mastumah is the latest setback in Idlib province for the government of President Bashar al-Assad, which lost control of the provincial capital in March and the stronghold of Jisr al-Shughur in April.

Pro-Assad forces withdrew to the town of Ariha further south, which, along with the Abu Duhur military airport and a few small government positions, make up the last remaining government military presence in Idlib province.

'Defensive positions'

A Syrian military source told AFP that clashes were ongoing in Al-Mastumah camp and the nearby town of the same name.

Syrian state television said the army was "taking defensive positions" outside of Ariha, implying that army units had withdrawn from Al-Mastumah itself.

Meanwhile separately, the Islamic State group attacked the Druze village of al-Haqef in Sweida province on Tuesday and clashed with pro-government forces north of the ancient city of Palmyra, the monitoring group said.

According to the Observatory, IS controls a series of villages in the northeast of Sweida province, the rest of which remains in government hands, and has tried to seize Khalkhalah military airport.

"There are clashes at the western entrance of Palmyra this morning, which lies on the road that leads to Homs city," provincial governor Talal Barazi told AFP.

Iran proud of Hezbollah gains

Meanwhile, a senior Iranian official said on Monday Tehran was proud of its key ally Hezbollah for advances it has made against Syrian rebels near the Lebanese border.

Ali Akbar Velayati, foreign affairs adviser to Supreme Leader Ayatollah Ali Khamenei, in a trip to Beirut praised the "victories" by the Shiite Hezbollah movement.

"We are filled with pride and appreciation when we see that in recent days, the valiant Lebanese resistance (Hezbollah) has achieved great progress and excellent victories alongside the brave Syrian army," he said after meeting Lebanon's parliamentary speaker.

Hezbollah, which is heavily backed by Iran, has been fighting to secure the Qalamun region that straddles the Syria-Lebanon border.

The region was largely recaptured by pro-Assad forces last year, but some rebels remain in the mountainous area along the frontier and have launched attacks into Lebanon.

Both Iran and Hezbollah are strong allies of Assad and have backed his forces throughout the uprising that began in March 2011.

Assad on Tuesday praised Iran's support as a "key pillar" in his fight against rebels, as a third Iranian official visited Damascus in less than a week.

"The support given by Iran to the Syrian people constitutes a key pillar in the battle against terrorism," official news agency SANA quoted Assad as saying during a meeting with Velayati.

Syria's government refers to all those seeking to oust Assad as "terrorists".

Syria war crimes justice unlikely despite evidence

Rights groups have steadfastly documented atrocities committed on the ground, and on Wednesday a committee of renowned investigators said it had enough evidence to prosecute up the chain of command to Assad himself.

But while those dossiers, collected by the Commission for International Justice and Accountability (CIJA) including former investigators from international tribunals, claim for the first time to be "trial ready," the world is not.

For political reasons, there is no court able to judge the crimes committed in a conflict that has killed more than 220,000 people, including at least 67,000 civilians and 11,000 children.

"It's very likely that no conflict has ever received as much investigatorial attention of mass atrocities, war crimes and crimes against humanity, with so little justice," said London-based international law expert Mark Kersten.

The International Criminal Court, which judges the world's most serious crimes, has no jurisdiction in Syria as it is not a member and ally Russia is expected to block the UN Security Council from authorising an ICC investigation.