While a cease-fire in Syria is supposed to take place by week's end, the fighting is escalating and new parties, such as Saudi Arabia, are preparing to enter the fray.
The recent developments come after the United States and Russia brokered a truce agreement early Friday in Munich for a "cessation of hostilities" in Syria's long-running civil war that could begin in a week.
On Sunday, Turkey announced the arrival of Saudi military jets that are poised to join the U.S.-led coalition against Islamic State militants in Syria. Turkey and Qatar are also considering sending ground troops into the fight, according to the Andalou News Agency.
In a letter to the United Nations Security Council on Sunday, Syria's Foreign Ministry accused Turkey of shelling government forces in northern Syria and sending military supplies accompanied by gunmen into its country, according to the Al Arabiya television network.
Pickup trucks mounted with heavy machine guns were "accompanied by around 100 gunmen some of whom are believed to be Turkish forces and Turkish mercenaries," the Syrian Foreign Ministry said.
Turkish artillery continued attacks for a second day Sunday on Syrian government forces and a Kurdish militia near the Turkish border.
Turkey has fired on the Kurdish People's Protection Units, known as YPG, in the past and considers it a threat because it is linked to the PKK, a U.S.-designated terrorist group that fought a yearslong insurgency for Kurdish independence in Turkey. But the YPG has received U.S. assistance in its fight against the Islamic State, also known as ISIL or ISIS.
State Department spokesman John Kirby on Saturday urged both Turkey and the YPG to step back.
"We have urged Syrian Kurdish and other forces affiliated with the YPG not to take advantage of a confused situation by seizing new territory," Kirby said. "We have also seen reports of artillery fire from the Turkish side of the border and urged Turkey to cease such fires."
Christy Delafield, a spokeswoman for Mercy Corps, a humanitarian aid group, said Sunday that the group's efforts to deliver food to residents in the besieged northern Syrian city of Aleppo "are threatened by an even greater escalation of violence in the last two days."
Since Friday, the group has delivered food kits to support 31,000 people for a month and enough flour to supply bread for 61,000 in the Azaz district, she said.
The cease-fire should be implemented immediately, and not after giving fighters time to press their offensives, said Dalia al-Awqati, Mercy Corps director of programs for north Syria.
"Each delay places innocent civilians at greater risk and impedes our efforts to support the half a million people who depend on us for food and other essential supplies," al-Awqati said.
Whether the cease-fire takes place and holds is an open question, considering the dominant role that Russian airstrikes now play in the Syrian conflict.
Syrian government and Kurdish forces are moving under Russian air cover toward encircling the eastern half of Aleppo, which has been under rebel control since 2012.
Strobe Talbott, president of the Brookings Institution think tank, said Sunday that it's difficult to see how Russia's role in the Syrian conflict is constructive as long as its priority is protecting Syrian President Bashar Assad and his regime.
"As long as the Assad regime is in place, as long as Assad himself is in place, it's almost impossible to imagine a peace and very hard to imagine a staunching of refugees pouring out of the country,"Talbott said. The result "is going to keep the war going."
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