The cessation of hostilities was initially to last for two days but was later extended until Tuesday at 00:01 am (21:01 GMT Monday).

Announcing a further prolongation the army command said: "The 'regime of silence' in Aleppo and its province has been extended by 48 hours from Tuesday 01:00 am (local time) to midnight on Wednesday."

The rebels had yet to confirm the extension of the truce which was decided after nearly 300 people were killed in an uptick in fighting in Syria's largest city since late April.

The announcement came as Russia and the United States agreed to boost efforts to find a political solution to the five-year war which has killed more than 270000 people and displaced millions.

"The Russian Federation and United States are determined to redouble efforts to reach a political settlement of the Syrian conflict" a joint US-Russian statement published by the Russian foreign ministry said.

The two powers also agreed to try extend a February 27 ceasefire across the whole of the country.

The ceasefire which was brokered by Washington and Moscow and excluded jihadist rebel groups fighting President Bashar al-Assad's forces was greatly strained by the upsurge in violence in Aleppo.

Washington and Moscow on Monday hailed some "progress" in reducing the fighting but admitted to ongoing "difficulties" in achieving a de-escalation in some areas as well as in ensuring humanitarian access to besieged areas.

On Sunday Syrian rebels fired rockets into a regime-held district of Aleppo killing five civilians including two children the Syrian Observatory for Human Rights said.

The Britain-based monitor also reported 10 civilians killed on Monday by regime bombardments in the northwestern province of Idlib which is controlled by Al-Nusra Front.

In the statement from Moscow Russia and the US restated their commitment to the ceasefire and vowed "to intensify efforts to ensure its nationwide implementation."

"We also intend to enhance efforts to promote humanitarian assistance to all people in need" they said.

To this end Russia "will work with the Syrian authorities to minimise aviation operations over areas that are predominantly inhabited by civilians or parties" to the ceasefire they added.

Washington meanwhile said it would step up assistance to its allies in the region "to help them prevent the flow of fighters weapons or financial support to terrorist organisations across their borders".

UN-brokered talks on the conflict held in Geneva fell apart three weeks ago when Syria's main opposition High Negotiations Committee (HNC) suspended its formal participation.

In telephone talks Monday US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov stressed "the need to pursue negotiations between Syrian authorities and all the opposition under UN mediation" according to the Russian foreign ministry statement.

- Parallel efforts -

The fate of Syrian President Bashar al-Assad is a major stumbling block in efforts to end a half-decade of war.

In Paris France called for the Syrian government and rebel forces to return to the negotiating table in Geneva "as soon as possible".

Speaking after a meeting with several Arab and Western backers of the Syrian opposition French Foreign Minister Jean-Marc Ayrault also called for "concrete guarantees on the maintenance of the ceasefire" and access for humanitarian aid on the ground.

Ayrault met with his Saudi Qatari Turkish and UAE counterparts in Paris. Kerry attended the talks but sources said he played a low-key role in a sign that Washington still believes the best hope for progress is to work most closely with Moscow.

Arriving for talks with Ayrault Kerry held up the joint US-Russian statement as "a commitment by Russia" to try rein in Assad's bombardments.

"But again the proof will be in the eating of the pudding not the making and we'll have to see what happens" he said.

A diplomatic source said it had taken a great deal of effort to persuade Kerry to attend because he did not want to upset the Russians who were strongly opposed to the meeting.

The United Nations has sought in vain for months to find a diplomatic solution to the conflict which has also forced millions to flee.

Also on Monday an audio message emerged in which the son of Al-Qaeda's late founder Osama bin Laden urged jihadists in Syria to unite claiming that the fight would pave the way to "liberating Palestine".

"The Islamic umma (nation) should focus on jihad in Al-Sham (Syria) ... and unite the ranks of mujahedeen there" said 23-year-old Hamza in the undated message posted online.