Russia is temporarily halting airstrikes on "gunmen" in Syria to give rebel groups a chance to cut their ties with the local branch of al-Qaeda, Russian state news agency TASS reported on Wednesday.

The move comes amid repeated complaints by Syrian rebels - who have signed up to a shaky cessation of hostilities - that Russian airstrikes have targeted their positions around the key northern city of Aleppo.

Rebels on Sunday issued an ultimatum to the United States and Russia, as sponsors of the truce, to end regime attacks and blockades in the Damascus area within 48 hours.

The US issued a public plea to the rebels to stick to the cessation of hostilities, saying that it was raising their concerns with Moscow, whose air force has been providing key backing to Syrian President Bashar al-Assad's overstretched forces.

Russia insists that its intervention in Syria is targeting the Islamic State extremist group and the al-Nusra Front, the Syrian branch of al-Qaeda, both of which are excluded from the truce.

However, Russian strikes in practice appear to have frequently targeted more moderate rebel groups.

Earlier this year, government forces, backed by Russian air power, cut rebels in Aleppo off from their main supply route north to the Turkish border.

The US has conceded that the battlefield alliances between more moderate rebel groups and al-Nusra have complicated implementation of the cessation of hostilities.

Al-Nusra, which is one of the strongest opposition forces on the ground, has spearheaded a recent rebel offensive in northern Syria, including areas just south of Aleppo.

Russia, the US and other countries backing Syria's warring parties last week warned that any side breaking the truce or blocking aid shipments would face consequences.

On Tuesday, the Russian military also called for a 72-hour ceasefire in Syria between government and rebel forces in areas on the outskirts of Damascus, where government troops have made some advances.

Fighting between rival rebel groups in Damascus' suburban region of Ghouta has left around 500 people dead in recent weeks, according to activists.

The fighting has pitted the Army of Islam - one of the largest rebel forces, whose stronghold is the Ghouta town of Douma - against rival group al-Nusra.

The Eastern Ghouta is strategically vital as it is the largest remaining opposition-held area in the Damascus region.

On Wednesday, the Army of Islam and the rival rebel group Faylaq al-Rahman announced an agreement to stop fighting between them.

In northern Syria, the powerful Kurdish-led forces reportedly made some advances in areas controlled by the Islamic State north of the militant group's de facto capital of al-Raqqa.

"Our fighters made advances in the area of Namourdiyeh, south of the village of Ain Issa," Brigadier Talal Silo, a spokesman for the Democratic Forces of Syria (DFS), said, referring to a village located north-west of al-Raqqa.

"The fighting is ongoing, but we know the advances will be limited in the early stages of this campaign as the other side will try to fight fiercely for every metre it controls," Silo told dpa.  

On Tuesday, the DFS announced the start of a military campaign  towards al-Raqqa, with the backing of a US-led air coalition. 

Kurdish officials said the the first stage of the offensive would not aim at entering al-Raqqa itself, but is designed to put pressure on the extremists.

The DFS is a coalition proclaimed last year by the Kurdish People's Protection Units (YPG) and smaller allied Arab groups.

The Syrian Observatory for Human Rights, a monitoring group, said the fighting is mainly focusing on areas on the edge of the village of Ain Issa.

The head of the Britain-based watchdog, Rami Abdel-Rahman, said dozens of families were forced to flee the area as a result of the US-led airstrikes on al-Raqqa and its outskirts, as well as the DFS onslaught.

On May 20, the US-led coalition dropped flyers over al-Raqqa, calling on civilians to leave.

In recent months, Islamic State has suffered major military setbacks and lost ground in Syria and neighbouring Iraq.

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