Tens of thousands of people protested on Wednesday in a town in northern Syria against Turkey, as speculation mounts it is planning to attack the Kurdish-held border region.

The demonstrators took to the streets of Afrin wrapped in flags of the Syrian Kurdish People's Protection Units (YPG) and carrying portraits of Abdullah Ocalan, the leader of the Kurdistan Workers' Party (PKK) jailed in Turkey.

"No to Turkish intervention," they chanted, according to images of the demonstration shared by activists and the Syrian Observatory for Human Rights monitoring group.

"All the people of Afrin, both Arabs and Kurds, have responded to the call of the Kurdish parties to protest... to express their opposition to the Turkish intervention," said Suleiman Jaafar, a Kurdish official in Aleppo province.

They wanted to show "their support for the border protection forces and our forces on the front lines to protect our lands," said Jaafar.

Turkish troops and YPG forces have repeatedly exchanged cross-border fire in recent days, leading to the deaths of three civilians on Tuesday, and there is speculation Ankara may be planning an assault on the group in Afrin.

"Since the beginning of the Syrian crisis, not a shot has been fired at Turkey from Afrin," Jaafar said, accusing Ankara of "shooting at us".

The Turkish newspaper Sabah said on Sunday that pro-Ankara Syrian rebels were on standby for an operation against the YPG and Russia could ensure security in the air.

Asked about the possibility of an operation around Afrin, presidential spokesman Ibrahim Kalin said Saturday: "We take all measures to protect our borders and national security."

He said the Turkey would "instantly" hit back against any threat from Syria, be it from IS, the PKK or the YPG.

Ankara considers the YPG a terrorist group and the Syrian branch of the PKK which has waged an insurgency inside Turkey since 1984.

But the United States is openly arming the YPG as it considers the Kurdish militia group the most effective fighting force against jihadists in Syria.

Turkey fears the creation of a continuous Kurdish entity in northern Syria stretching from the Iraqi border almost too the Mediterranean.

It fears this could encourage separatism amongst its own Kurds, especially as the Iraqi Kurdistan region is planning a referendum on independence.