New recruits attend a military training to be part of the Free Syrian Army in eastern al-Ghouta, near Damascus. Picture: Ammar al-Bushy

Beirut - The Free Syrian Army has sacked its military chief after the Western-backed rebels suffered battlefield setbacks, amid signs the warring parties are escalating the fighting that has already killed more than 140 000 people.

Activists warned on Monday that regime troops are preparing a ground offensive against the town of Yabrud, the last rebel stronghold in the strategic Qalamoun region near Lebanon's border, after days of aerial bombardments.

On the diplomatic front, US Secretary of State John Kerry slammed Russia on Monday for “enabling” Syria's President Bashar al-Assad to stay in power, after Geneva peace talks broke off without result at the weekend.

Moscow quickly dismissed the allegation and launched its own broadside, accusing the US of failing to ensure a “truly representative opposition delegation” attended the talks.

The sacking of rebel military commander Selim Idriss was announced on Sunday by the FSA, which said he was being replaced by Brigadier General Abdelilah al-Bashir, an army deserter.

Colonel Qassem Saadeddine of the rebel coalition said the decision was taken due to “the paralysis within the military command these past months” and the need to “restructure”.

A source inside the Syrian opposition told AFP that Idriss - who was appointed to the role in December 2012 - had faced criticism for failings on the battlefield.

These included “errors and carelessness in combat” and “poor distribution of weapons” among the rebels on the ground, the source said.

The FSA has taken a beating on the battlefield in recent months not only by regime troops but also by Islamist fighters who have joined the battle to unseat Assad.

Considered the “moderate” rebel group, the FSA was once the country's strongest armed opposition force but is now increasingly marginalised by Islamists, including al-Qaeda-linked groups.

Analysts said that with the failure of the latest round of peace talks, Syria's regime and rebels are likely to ratchet up military pressure on the ground.

“I fear that the failure of the Geneva talks will lead to military escalation - it will probably get worse before it gets better,” said Volker Perthes, director of the German Institute for International and Security Affairs.

“Both sides will try to show that they can change the balance on the ground in their favour, and that they aren't forced to negotiate out of weakness.”

The Syrian Observatory for Human Rights monitoring group reported a surge of fighting Monday around Yabrud, which lies on a strategic highway linking Damascus and the central city of Homs.

The head of the Britain-based Observatory, Rami Abdel Rahman, told AFP that the Syrian air force dropped explosive-packed barrels on the outskirts of Yabrud, and that fighting had erupted in the nearby rebel-held towns of Ras al-Maara and Al-Sahel.

Pro-regime newspaper Al-Watan reported that the troops were battling jihadists around Yabrud, including fighters from Al-Nusra Front, al-Qaeda's branch in Syria.

Activists say the regime forces are being helped in the Yabrud offensive by fighters from the Lebanese Shi'a movement Hezbollah.

Hezbollah chief Hassan Nasrallah on Sunday vowed that his group would crush the Syria rebels.

“We will win this battle, God willing,” he said in a speech broadcast on Lebanese television. “It is a question of time.”

The flare-up in fighting comes as diplomatic efforts to try to end the Syrian war seem to have ground to a halt.

A second round of US and Russian-backed peace talks between regime representatives and the opposition broke down on Saturday in Geneva with no date set for a third round.

US Secretary of State John Kerry said in Jakarta that the Damascus regime “stonewalled” in Geneva with “increased support from Iran, from Hezbollah and from Russia”.

“Russia needs to be a part of the solution and not be distributing so much more weapons and so much more aid that they're in fact enabling Assad to double-down. Which is creating an enormous problem.”

Russian Foreign Minister Sergei Lavrov hit back, telling reporters in Moscow that the Syrian opposition delegation at the Geneva negotiations was not represented by some of the most important members of the National Coalition umbrella group.

“They (Washington) assured us that they will be doing everything possible to ensure there is a truly representative opposition delegation,” Russia's top diplomat said.

“For now, they have been having trouble doing this.” - AFP