The Turkish army has shelled Kurdish militia in northern Syria for a second day, while Russia made clear it would continue bombing Syrian rebel targets, raising doubts that a planned ceasefire will bring much relief.
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 meanwhile 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 hub before the conflict.
The situation 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.
Efforts to deliver humantitarian aid were being threatened by the latest escalation of violence.
The Kremlin said Russian President Vladimir Putin and US President Barack Obama had agreed to intensify cooperation to implement the agreement on Syria struck in Munich.
After a phone call between Putin and Obama, confirmed by the White House on Sunday, the Kremlin said both gave a "positive valuation" to the Munich meeting.
But the Kremlin statement made clear Russia would continue bombing raids against Islamic State and "other terroristic organisations", an indication that it would also be targeting groups in western Syria where jihadists such as al-Qaeda are fighting President Bashar al-Assad in close proximity to rebels deemed moderate by the West.