Turkey continued on Tuesday its military operation against the Kurdish People’s Protection Units (YPG) in Afrin in northern Syria, while Washington called on Ankara to exercise restraint. The Kurdish-led authorities in the area meanwhile called for mass mobilization in defense of Afrin against Turkey’s offensive. “The will of the people cannot be vanquished,” the civilian administration said in a statement. “We call on all our noble people to defend Afrin and its pride, and to contribute in all related activities.” The Syrian Observatory for Human Rights reported that fierce battles had taken place in Afrin on the fourth day of the offensive. Turkey’s Operation Olive Branch has been criticized by the United States and European Union that interpreted it as Ankara opening a new front in the Syrian conflict. US Defense Secretary Jim Mattis urged Turkey to exercise restraint in its military operations. “We seriously take Turkey’s security valid concerns… but in the Afrin area, we had actually gotten to the point where humanitarian aid was flowing, refugees were coming back in ... The Turkish incursion disrupts that effort,” he said. Qatar meanwhile voiced its support for the Turkish offensive, saying that it is aimed at defending its national security. "The state of Qatar reaffirmed its support for the efforts of the republic of Turkey to maintain its national security in the wake of the breaches and terrorist attacks carried out inside Turkish territories," Foreign Ministry spokeswoman Lolwa Al-Khater said on Monday. It also stems from its concern to protect Syrian territories from the threat of separatism. Turkey views the YPG as a terror group and an offshoot of the Kurdistan Workers Party (PKK), which has long fought for autonomy. The Observatory said that since the beginning of the offensive some 60 YPG and Ankara-backed Syrian opposition faction members have been killed.