Turkey has serious worries about the plan to halt violence in Syria because of the fighting on the ground, President Tayyip Erdogan’s aide and spokesman said on Friday, hours before the U.S.-Russian deal was due to start.

Ibrahim Kalin also said that while Turkey had no plans for a unilateral ground operation in Syria, it would respond to any security threats in line with its rules of engagement.

Fighting has continued across much of western Syria, with heavy air strikes reported to have hit rebel-held areas to the east of Damascus. The “cessation of hostilities” agreement is due to take effect at midnight (2200 GMT on Friday).

“We support this ceasefire in principle, but unfortunately we have serious concerns about the future of this ceasefire as the fighting goes on,” Kalin told a news conference.

The Syrian government has agreed to the plan. The main opposition alliance, which has deep reservations about the terms, has said it is ready for a two-week truce to test the intentions of the government and its Russian and Iranian backers.

NATO member Turkey’s role in the ceasefire has been complicated by its deep distrust of the Washington-backed Syrian Kurdish YPG. Ankara sees the group as a terrorist organisation and has shelled YPG positions in northern Syria in recent weeks in retaliation, it says, for cross-border fire.

Washington, which says the YPG is not a terrorist organisation, has supported the group in the fight against Islamic State (ISIS) in Syria.

The YPG told Reuters this week it would respect a ceasefire but reserved the right to respond if attacked.

Kalin also said Saudi planes – due to take part in air strikes against ISIS – had started arriving at Turkey’s Incirlik air base.