US President Barack Obama urged Russia on Sunday to stop bombing "moderate" rebels in Syria in support of its ally President Bashar Al Assad, a campaign seen in the West as a major obstacle to latest efforts to end the war.

Major powers agreed on Friday to a limited cessation of hostilities in Syria but the deal does not take effect until the end of this week and was not signed by any warring parties - the Damascus government and numerous rebel factions fighting it.

Russian bombing raids directed at rebel groups are helping the Syrian army to achieve what could be its biggest victory of the war in the battle for Aleppo, the country's largest city and commercial centre before the conflict.

There is little optimism that the deal reached in Munich will do much to end a war that has lasted five years and cost 250,000 lives.

The Kremlin said President Vladimir Putin and Obama had spoken by telephone and agreed to intensify co-operation to implement the Munich agreement.

But a Kremlin statement made clear Russia was committed to its campaign against Islamic State and "other terrorist organisations", an indication that it would also target groups in western Syria where jihadists such as al Qaeda are fighting Assad in close proximity to rebels deemed moderate by the West.

Russia says the "cessation" does not apply to its air strikes, which have shifted the balance of power towards Assad.

It says Islamic State and the al Qaeda-affiliated Nusra Front are the main targets of its air campaign. But Western countries say Russia has in fact been mostly targeting other insurgent groups, including some they support.

The White House said Obama's discussion with Putin stressed the need to rush humanitarian aid to Syria and contain air strikes.

"In particular, President Obama emphasized the importance now of Russia playing a constructive role by ceasing its air campaign against moderate opposition forces in Syria," the White House said in a statement.

Relief workers said efforts to deliver humanitarian aid were being threatened by the latest escalation of violence.

"We must ask again, why wait a week for this urgently needed cessation of hostilities?" said Dalia al-Awqati, Mercy Corps director of programs for North Syria.

The situation in Syria has been complicated by the involvement of Kurdish-backed combatants in the area north of Aleppo near the Turkish border, which has drawn a swift military response from artillery in Turkey.

The Kurdish YPG militia, helped by Russian air raids, seized an ex-military air base at Menagh last week, angering Turkey, which sees the YPG as an extension of the PKK, a Kurdish group that waged a bloody insurgent campaign on Turkish soil over most of the past three decades.

Turkey began shelling while demanding that the YPG militia withdraw from areas it has captured from Syrian rebels in the northern Aleppo region in recent days, including the Menagh air base. The bombardment killed two YPG fighters, according to the Syrian Observatory for Human Rights.

The Syrian Kurdish PYD party rejected Turkish demands for withdrawal, while the Syrian government said Turkish shelling of northern Syria amounted to direct support for insurgent groups.

France called on Turkey to stop the shelling, but Turkey said it would continue to respond to Kurdish militia attacks in Syria.

Syria also said Turkish forces were believed to be among 100 gunmen that entered Syria on Saturday with a dozen pickup trucks mounted with heavy machine guns in an operation to supply rebel fighters.

Other fronts were also active on Sunday.

Kurdish-backed forces were fighting with insurgent groups near Tel Rifaat in the northern Aleppo countryside, while farther south, government forces renewed their shelling of rebel positions to the northwest of Aleppo city.

The Syria Democratic Forces alliance, which includes the YPG, gained more ground from insurgents north of Aleppo, capturing a village on the road between the two rebel-held towns of Tel Rifaat and Azaz, the Observatory reported.

It also reported air strikes by jets believed to be Russian in areas east of Damascus, north of Homs, and in the southern province of Deraa.

Reaction from politicians in the West to the Munich deal was sceptical.

US Senator John McCain said he did not view the deal as a breakthrough. "Let's be clear about what this agreement does. It allows Russia's assault on Aleppo to continue for another week," he said at security conference in Munich.

"Mr Putin is not interested in being our partner. He wants to shore up the Assad regime," McCain said.

A senior ally of German Chancellor Angela Merkel said Russia had gained the upper hand in Syria through armed force.

Norbert Roettgen, head of the foreign affairs committee in the German parliament, said Russia was determined to create "facts on the ground", to bolster its negotiating position.

As the fighting continued, the Syrian army urged citizens in Deraa province, the Ghouta area east of Damascus, and in rural districts east of Aleppo to quickly seek "reconciliation" with the government.

So-called local reconciliation agreements are often seen as a means for the government to force surrender on insurgents, and have typically followed lengthy blockades of rebel areas and the civilians living there.

Saudi Arabia confirmed it had sent aircraft to Turkey's Incirlik air base to join the fight against Islamic State, but said any move to deploy Saudi special forces into Syria must await a decision by the US-led coalition combating the militants.

Any ground operations in Syria will lead to "a full-fledged, long war", Russian Prime Minister Dmitry Medvedev warned.-Reuters