The US will set a date on Friday for Syria's warring parties to return to the negotiating table for a second round of talks, UN Syria envoy Staffan de Mistura said on Thursday, Reuters reported.

De Mistura abruptly aborted a first round of talks on February 3, saying more preparatory work was needed from the "International Syria Support Group" (ISSG), led by the United States and Russia.

Since then Syrian government forces, backed by Russia and Iran, have continued a major offensive against rebels around the city of Aleppo, and President Bashar al-Assad's government and the opposition have both said they could support a US-Russian proposal for a "cessation of hostilities."

De Mistura said he would brief the UN Security Council on Friday and then announce a date for a new round of talks, but he declined to comment further.

"I will announce tomorrow when the parties (will reconvene) and the new talks (will happen)," he told reporters, as he arrived for a meeting of the ISSG task force on humanitarian access.

Another ISSG task force, on cease-fires, will convene for the first time in Geneva on Friday, De Mistura said - an opportunity for diplomats from ISSG countries such as Saudi Arabia, Turkey and Iran to say whether they endorse the US-Russian proposal.

Combatants are required to say whether they will agree to the "cessation of hostilities" in the five-year war by noon on Friday, and to halt fighting on Saturday.

Turkish Prime Minister Ahmet Davutoglu on Thursday said the cease-fire agreed between Syria's warring parties was only valid inside Syria and was "not binding" for Turkey if its national security was threatened, AFP reported on Thursday.

"It must be known that the cease-fire is valid in Syria," Davutoglu said in televised remarks in the central Anatolian province of Konya.

"When it is a question of Turkey's security, then the cease-fire is not binding for us," he added.

Turkey on successive days last week targeted Kurdish fighters inside Syria with artillery barrages, saying that the army was responding to incoming fire, and had repeatedly reserved the right to open fire again.

Davutoglu said Turkey would closely monitor how the cease-fire would be implemented, adding that "we support the cease-fire under any circumstances."

Turkey has demanded that the Syrian Kurdish Democratic Union Party (PYD) and its People's Protection Units (YPG) militia should remain outside the scope of the truce.

He warned the YPG and the PKK "not to support terror in Turkey" just because of the cease-fire in Syria.

Russia's foreign ministry spokeswoman Maria Zakharova said on Thursday that the Syria cease-fire process was underway despite what it said were attempts by some US officials to sabotage it, Reuters reported.