As the USA tries to keep the focus on the fight against ISIS - which has been routed out of its former Syrian strongholds - tensions are growing between Kurdish and Turkish forces, both of which are allied with the Americans.
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The Syrian government also has been pressing its campaign in a rebel-held area near the capital of Damascus.
President Bashar al-Assad's regime has recently ramped up attacks against the opposition, with USA officials accusing his forces of using chemical weapons such as chlorine in recent weeks in Eastern Ghouta and Idlib, another rebel stronghold.
A fourth consecutive day of heavy regime bombing raids on the rebel-held enclave of Eastern Ghouta near Damascus killed 22 civilians on Thursday, a monitor said.
Russia's U.N. Ambassador Vassily Nebenzia said he will raise the coalition's airstrike during closed consultations later Thursday in the U.N. Security Council. The U.S. -led coalition said its action in eastern Syria was in "self-defense", citing a major attack on its allied forces and U.S. advisers in Deir el-Zour province.
At least 100 pro-regime fighters were killed, a United States military official said.
Russia's military bases in western Syria were also hit last month in a series of drone attacks, challenging Moscow's gains in the country still torn by conflict. The Syrian Observatory for Human Rights put Tuesday's death toll at 70 in what it said was the highest it had recorded since December. The civilian death toll from the Russian and Syrian government siege included 63 children and 72 women. The monitoring group did not mention allegations of chlorine use in Tuesday's attacks.
Russia, an ally of Syrian President Bashar al-Assad whose military support has helped government forces claw back territory, on Thursday said a ceasefire was unrealistic. "We demanded it several times".More news: My Nintendo Gold Points can soon be spent on the Switch eShop
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It was the second attack in a week on Turkish soldiers trying to establish the position, near the front line between rebels and pro-Syrian government forces.
"It is like the end of days", said Raed Saleh, the head of the first-responders known as White Helmets, describing the last 24 hours of attacks on the opposition-held eastern Damascus suburb of Ghouta and northwestern Idlib province.
The intensity of the air strikes, which have killed more than 220 civilians in just four days, has made it extremely hard for relief workers to assist the estimated 400,000 people who live in the enclave under siege, CARE International said.
"The commission remains committed to fulfilling its mandate to investigate and document all human rights violation (s) occurring in the context of the Syrian armed conflict, regardless of who commits them", Pinheiro said. The U.N.is now investigating the most recent attacks, though Russian Federation blocked a statement originating in the Security Council condemning Assad's use of the forbidden weapons.
"We are particularly concerned about attacks against civilians and civilian objects, such as hospitals".
"We would like to see a cease-fire, the end of the war, but the terrorists, I am not sure, are in agreement", Russia's United Nations envoy, Vassily Nebenzia, told reporters.
In agreement with Iran and Russian Federation, the Turkish military is setting up observation posts in parts of Idlib and Aleppo province.
Syria's state media say the US -led coalition has bombed government-backed troops in eastern Syria, citing reports of dozens killed and wounded.
United Nations officials in Syria called for the cessation of hostilities to enable humanitarian aid deliveries, and the evacuation of the sick and wounded, listing seven areas of concern including northern Syria's Kurdish-led Afrin region, being targeted by a Turkish offensive.