UNITED NATIONS - U.N. Secretary-General Antonio Guterres called Monday for the "immediate de-escalation" of the situation in northeastern Syria, as the humanitarian situation rapidly worsens with Turkey's military pushing farther into the area.

"The secretary-general is gravely concerned over the military developments in northeast Syria, which have already reportedly resulted in many civilian casualties and the displacement of at least 160,000 civilians," spokesman Stephane Dujarric told reporters. "The secretary-general calls for the immediate de-escalation and urges all parties to resolve their concerns through peaceful means."

Turkey began a military incursion into northeastern Syria on Wednesday, targeting the Kurdish Syrian Democratic Forces (SDF), which Ankara considers as terrorists aligned with separatists inside Turkey. Western countries view the SDF as a key ally in the fight against the self-described Islamic State terror group.

While Guterres did not call for the offensive to end, he urged "maximum restraint" and said any military operation must fully respect international law, including the U.N. Charter and international humanitarian law, as well as the protection of civilians.

The U.N. chief also called for safe and unhindered access for humanitarian workers, as aid agencies said access was becoming more difficult and dangerous.

U.S.-based charity Mercy Corps, which has been aiding civilians in northeast Syria since 2014, said Monday it is suspending its operations in that area and evacuating its international staff.

"This is our nightmare scenario, there are tens of thousands of people on the run and we have no way of getting to them," Made Ferguson, Mercy Corps' Deputy Country Director for Syria, said in a statement. "We just cannot effectively operate with the heavy shelling, roads closing, and the various and constantly changing armed actors in the areas where we are working."

Mercy Corps said it would make every effort to return as soon as possible.

The United Nations says 1.8 million of the three million people living in northeastern Syria were already in need of assistance and the current military operation will only aggravate an already dire humanitarian situation.

Among the residents, the U.N.'s agency for children (UNICEF) estimates nearly 70,000 children are among those displaced since last week.

"UNICEF confirms that at least four children have been killed and nine others injured in northeast Syria," UNICEF Executive Director Henrietta Fore said in a statement. "Seven children have reportedly also been killed in Turkey," she added.

The World Food Program has been able to assist more than 70,000 people fleeing towns in the area, but said it is deeply concerned about the safety of civilians caught in the violence. The food agency said it is "vital" that supply routes be kept open and safe for aid deliveries.

The International Committee of the Red Cross reported that tens of thousands of people are fleeing villages and towns along the Syrian-Turkish border and warned that the displaced could swell to as many 300,000. The ICRC also expressed concerns that civilian infrastructure including dams and water stations are in the area of hostilities and, if damaged, could cause severe water shortages.