In their bid to overthrow President Bashar al-Assad the armed rebels of Syria are stepping up a war of attrition against the regime and trying to push more officials to defect, experts say.

"The (rebel) Free Syrian Army has established a strategy based on depriving the enemy of its cadres, by encouraging defection from the regime, while weakening it militarily through guerrilla warfare," said Riad Kahwaji, head of the Institute for Near East and Gulf Military Analysis.

After Prime Minister Riad Hijab joined the opposition this week, FSA spokesman Kassem Saadeddine said that "one of the opposition's main objectives, in order to bring down the regime, is to encourage defections."

"After Hijab's defection, we have received several calls from senior officers and officials, asking us to help them flee," he said via Skype.

Hijab was the highest-ranking official to defect since an anti-regime revolt broke out in March 2011.

He announced breaking ranks with the Syrian regime on August 5 but only arrived in neighbouring Jordan two days later, according to authorities in Amman.

An amateur video posted on YouTube on Tuesday showed Hijab seated on the floor of a house, surrounded by members of his family and activists, a clear confirmation he was under FSA protection.

"The FSA helped him cross the border," Saadeddine said, without giving details of the operation.

Hijab's defection shows that "the regime is disintegrating," said Abdel Basset Sayda, head of Syria's main opposition coalition, the Syrian National Council (SNC).

Washington agreed.

National Security Council spokesman Tommy Vietor said reports of the defection of Hijab and senior officials were "just the latest indication that Assad has lost control of Syria and that the momentum is with the opposition forces and the Syrian people."

"It's clear that these defections are reaching the highest levels of the Syrian government and demonstrate that the Syrian people believe Assad's days are numbered."

Military expert Jeffrey White, of the Washington Institute for Near East Policy said it took "some kind of network to make this (defection) happen. You have to have a system, a way of getting in touch and communicating two ways. That could be electronic, but also trusted agents."

He noted the difficulties involved in ensuring that whole families escape safely, saying it requires more than "some guy throwing his family in the car."

"There has to be some physical process to get the person or multiple persons to the border. Someone at the border to receive them or escort them in transit," said White.

The SNC has made multiple calls to regime troops and officers, as well as intelligence agents and diplomats, to defect, a member of the opposition group said.

"We expect the wave (of defections) to pick up speed," the member said.

Speaking on condition of anonymity, an Arab diplomat said Saudi Arabia, Kuwait and Qatar set up a $15 billion fund in May to help encourage regime figures to defect, in a bid to see Assad out by the end of 2012.

On the ground, the rebels seek to stretch the regular army's forces as far as possible, by opening multiple fronts at once.

"There is a war between a militia that is trying to expand its operations over the whole of Syria's territory on one hand, and a strong regular army on the other," said Kahwaji.

"The principle of a guerrilla is war of attrition."

At this stage of the conflict, the rebels, said Kahwaji, are taking the initiative. "The rebels choose the time and the place of the outbreak of each new battle," he added.

The FSA launched the battle to "liberate" Damascus before withdrawing from the capital, after three weeks of fighting. The rebel army then kickstarted the "battle for Aleppo," the country's second city and scene of raging combat.

At the same time, the FSA is staging operations in the northwest province of Idlib, Daraa in the south and Deir Ezzor in the east, targeting army convoys and regime checkpoints.

In the border area with Turkey, a journalist saw convoys of dozens of vehicles travelling under the cover of night.

A Syrian activist said the convoys carry "weapons from Turkey into Syria."

The opposition has said that Qatar, Saudi Arabia and Libya have provided the rebels with weapons, but the FSA says they are not the weapons they need.

"If we had weapons of quality, we would have already liberated the country," said Saadeddine.