DAMASCUS, Dec. 23 (Xinhua) -- Dozens of Turkish military vehicles headed on Sunday to frontline areas with the Kurdish militia in northern Syria, in what appeared to be preparations for an anticipated campaign against Kurdish fighters, a war monitor reported.

These military reinforcements were headed to the cities of Jarablus and Sajour in northern Syria, close to the Kurdish-controlled Manbij city in the northern countryside of Aleppo Province, said the Syrian Observatory for Human Rights.

According to the London-based watchdog, the Turkish forces would start from Manbij if an operation is launched, as Manbij is the area from which Ankara has for long asked the U.S. forces to remove the Kurdish People's Protection Units (YPG).

Turkey sees the YPG and its allied Syrian Democratic Forces (SDF) as separatists and terrorists, citing their links with the Kurdistan Workers' Party (PKK), which is listed as an terrorist organization by Turkey, the United States and the EU.

However, the U.S.-led anti-terror coalition has been backing the Kurdish militia in several battles, clearing the Islamic State (IS) militants from key areas in northern Syria, mainly the former IS de facto capital of Raqqa.

Turkish President Recep Tayyip Erdogan recently said his forces and allied rebel fighters in Syria would launch a campaign against the Kurdish militia in the eastern part of the Euphrates River in eastern Syria.

Afterward, the United States has announced its decision to withdraw from Syria, where it has around 2,000 troops largely embedded with the Kurdish militia in northern Syria.

Some commanders in the SDF were quoted by the Observatory in an earlier report as describing the decision of the United States to pull out of Syria as "a stab in the back."

The U.S. withdrawal will take around 100 days to complete, during which the anti-IS battles, which have been going on since September, are believed to continue.

Rami Abdul-Rahman, head of the Observatory, said the Kurdish-led forces may withdraw from eastern Deir al-Zour to protect their areas in the northern countryside of Aleppo on the heels of the Turkish military buildup there.

On the situation in northern Syria, the pro-government al-Watan newspaper said the YPG and SDF, as well as the U.S. forces in Manbij, are all on alert, fearing wide-scale protests in the city against the presence of the Kurdish militia there.

The Kurdish militia forces have been controlling areas in northern Syria since the early years of the crisis in Syria, in order to force Damascus to recognize a federal rule or autonomy for the Kurds in northern Syria.

However, Syrian Foreign Minister Walid al-Moallem said recently that Kurdish federalization or self-rule is out of the question.

Now with the withdrawal of the U.S. forces and the Turkish threats of military action, observers say the Kurdish militia will have to return under the umbrella of the Syrian state.