BEIRUT/AMMAN (Reuters) - Turkey-backed Syrian rebels launched an offensive into territory held by the Kurdish YPG militia north of the Syrian city of Aleppo on Saturday, seizing some territory before heavy shelling forced them to retreat.

The operation marked an escalation on one of the most complicated theaters of the multi-sided Syrian war. Though the rebels are targeting the YPG, Syrian government forces are also deployed nearby as are their Russian and Iran-backed allies.

The Turkey-backed Syrian National Army took three villages before withdrawing “because of heavy shelling and the lack of an ability to sweep the area completely in the light of the targeting of our forces”, said Yousef Hammoud, its spokesman.

He said pro-Damascus forces had shelled the advancing National Army fighters.

The YPG, which has fought alongside U.S. forces against Islamic State in eastern and northeastern Syria, has lost ground in the northwest since early 2018, when Turkish forces and their Syrian allies drove it from the Afrin region.

A military source in the Afrin Liberation Forces, which is close to the YPG, told Reuters the Turkey-backed rebels had advanced into an area where the Kurdish forces had no presence before being forced out.

“Now, after strikes from our forces, the opposition forces were forced to withdraw from those positions,” the source said.

The National Army was formed with Turkish backing from a number of rebel Free Syrian Army groups. Its main foothold is a chunk of territory northeast of Aleppo known as Euphrates Shield that is secured with help from Turkish forces on the ground.

The FSA groups have long vowed to take the YPG-held territory north of Aleppo including the town of Tel Rifaat, taken by the Kurdish militia since 2016.

The Turkish defense ministry said one Turkish soldier was killed and another was wounded in a YPG attack in Tel Rifaat on Saturday.

Turkey views the YPG as an extension of the Kurdistan Workers’ Party (PKK), which has waged an insurgency on Turkish soil for autonomy in Turkey’s largely Kurdish southeast since 1984. The PKK is deemed a terrorist organization by Turkey, the United States and the European Union.

The YPG is the backbone of the Syrian Democratic Forces (SDF), the main Syrian partner of the U.S.-backed coalition against Islamic State. The SDF controls northeastern and eastern Syria, approximately one quarter of the country.