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Rights groups and anti-war activists warned of a looming "humanitarian catastrophe" Wednesday as Turkish forces invaded northeastern Syria and launched airstrikes against Kurdish targets, forcing civilians to flee in panic.

The attack comes days after U.S. President Donald Trump gave his Turkish counterpart Recep Tayyip Erdoğan a green light to begin the operation by announcing the abrupt withdrawal of American forces from northeastern Syria.

The Kurdish-led Syrian Democratic Forces (SDF) warned in a statement Wednesday that Turkey's assault "will spill the blood of thousands of innocent civilians because our border areas are overcrowded."

On Twitter, SDF spokesman Mustafa Bali accused Turkey of deliberately targeting "civilian areas."

Turkish warplanes have started to carry out airstrikes on civilian areas. There is a huge panic among people of the region. — Mustafa Bali (@mustefabali) October 9, 2019

Fighters with the Kurdish People's Protection Units (YPG), which leads the SDF, told CNN that hundreds of civilians scrambled to escape northeastern Syria as Turkey began bombarding the area:

Hundreds of civilians in northern Syria have fled areas on the border with Turkey within the last 30 minutes, two YPG fighters and witnesses tell CNN. @npwcnn reports. https://t.co/Pm88pjHaON pic.twitter.com/iyoy1sjNcT — CNN Newsroom (@CNNnewsroom) October 9, 2019

In a statement issued before Turkey launched its attack, Amnesty International said "it is imperative that all parties to this conflict respect international humanitarian law, including by refraining from carrying out attacks on civilians and civilian objects, as well as indiscriminate and disproportionate attacks."

"The international community must take measures to ensure respect for international humanitarian law by the Turkish authorities and pro-Turkey armed groups and Kurdish forces if yet another humanitarian catastrophe in northern Syria is to be avoided," said Amnesty.

Misty Buswell, Middle East policy director at the International Rescue Committee (IRC), expressed deep concern about the impact Turkey's assault will have "on civilians, including our own staff members and their families, and the destabilizing effect this will have on a population that has already borne the brunt of the eight-year-long conflict in Syria."

"Many of these people have already been displaced multiple times and suffered horribly under the brutal rule of ISIS," said Buswell, "only to be facing yet another crisis."