The United Nations Security Council is due to vote on Thursday on rival draft resolutions that call for a truce in northwest Syria after Russia and China put forward their own text in response to a proposal by Kuwait, Germany and Belgium.

Syria's northwest corner, including the Idlib region, is the last major chunk of territory still in rebel hands after more than eight years of war.

Experts from the council's 15 members have met three times to negotiate on the latest text drafted by Kuwait, Germany and Belgium last month. A resolution needs nine votes and no vetoes by Russia, China, France, Britain or the United States to pass.

Russia and China put forward a proposal on Wednesday that "reaffirms that the cessation of hostilities shall not apply to military operations against individuals, groups, undertakings and entities associated with terrorist groups, as designated by the Security Council."

Syrian forces, aided by Russian air power, have been waging a five-month-long offensive in the Idlib region.

Shortly after the draft UN resolution was proposed by Kuwait, Germany and Belgium, Damascus declared a truce on August 31 that brought a lull in air strikes. However, Syrian troops on Sunday shelled south of Idlib, according to rescuers and residents.

The United Nations said that since the start of hostilities in northwest Syria in April, more than 550 civilians have been killed and some 400,000 people displaced. Almost half of the displaced people are living in open-air areas or under trees.

The leaders of Turkey, Russia and Iran meeting in Ankara on Monday agreed to try to ease tensions in the Idlib region, but disagreements between the countries appeared to linger, especially over the threat from Islamic State.

UN Secretary-General Antonio Guterres announced last month that the world body would investigate attacks on UN-supported facilities and other humanitarian sites in northwest Syria. Russia and Syria have said their forces are not targeting civilians or civilian infrastructure.

The locations of the UN-supported facilities and other humanitarian sites like hospitals and health centers had been shared with the warring parties in a bid to protect them. However, the United Nations has questioned whether they have been made targets.