Turkish air strikes pounded the Syrian border region of Afrin and fighting raged on two fronts as Ankara pursued its offensive against the Kurdish enclave on Tuesday.

A monitoring group and Kurdish sources said Turkey's air force had stepped up its raids on the 10th day of operation "Olive Branch", which sees Turkey providing air and ground support to Syrian opposition fighters in an offensive against Kurdish militia in northwestern Syria.

Ankara has pushed forward with the operation to force the Kurdish People's Protection Units (YPG) from the region despite international concerns and reports of rising civilian casualties.

Turkey has cracked down on criticism of the operation and on Tuesday detained all the top members of the country's main medical association, including its chief.

In reaction to the offensive, the Kurds were not attending peace talks Tuesday aimed at resolving Syria's almost seven-year civil war being held in the Russian city of Sochi.

Turkish jets were hitting Kurdish positions in the towns of Rajo and Jandairis, said the Syrian Observatory for Human Rights monitoring group.

Syrian rebels backed by Turkey "were engaged in fierce battles against Kurdish forces" in the two towns, said Rami Abdel Rahman, the head of the Britain-based Observatory, which uses a network of sources to monitor Syria's war.

"Turkey's aerial campaign against Afrin has escalated since Monday," he added.

A spokesman for the YPG, which Ankara considers a "terror" group, said the strikes had been relentless.

"Since yesterday, the bombardment by Turkish aircraft has not stopped in some areas," said Brusk Hasakeh.

It was unclear how many civilians remained in Rajo and Jandairis as many had already fled to Afrin town, the capital of the district.

An AFP journalist on Tuesday heard consecutive strikes hitting areas surrounding Afrin town.

The Observatory says at least 67 civilians have been killed since the start of the operation on January 20. Turkey strongly rejects such claims, saying it is doing everything possible to avoid civilian casualties in the operation.

At least 85 YPG militiamen have died, the Observatory says, as have 81 fighters from the rebel groups fighting with Turkish backing.

Turkey says seven of its soldiers have been killed.

Turkish state-run news agency Anadolu reported on Tuesday that two villages in the Afrin region had been "cleared" of the YPG.

Turkey and allied forces have made gains in the offensive and on Sunday seized control of Mount Barsaya, a strategically important high point near the town of Afrin.

A Turkish military convoy of dozens of vehicles crossed the border overnight, the Observatory said. It initially headed towards an area south of Afrin but was forced to change course after coming under fire from forces loyal to President Bashar al-Assad's regime.

Turkish relations with the United States have soured over Ankara's stance on the YPG -- which Ankara says is a "terrorist" offshoot of Turkey's outlawed Kurdistan Workers' Party (PKK).

The YPG has received support from the United States, with its fighters spearheading the battle against the Islamic State group across swathes of Syria.

Turkey's President Recep Tayyip Erdogan has threatened to expand the offensive against the YPG to other Kurdish areas including Manbij, east of Afrin.

Speaking at a meeting of lawmakers from his ruling Justice and Development Party (AKP), Erdogan said Tuesday: "We will not stop until we eliminate the terror threat from our border."

The arrests among members of the Turkish Medical Association (TTB) came after the organisation issued a statement saying that "war is a man-made public health problem".

The talks in Sochi had meanwhile been delayed by several hours, as Moscow struggled to bring together key players.

Syria's main opposition group, like the Kurds, said they would boycott the event, and last-minute negotiations were under way to bring others together at the table.

Few expect the congress, co-sponsored by Russia, Iran and Turkey, to make much progress in ending Syria's civil war, which has killed more than 340,000 people and devastated the country since breaking out in 2011.

The conflict was also still raging in the northern province of Idlib, where the Observatory said another eight civilians were killed in regime air strikes Tuesday.

In the village of Saraqeb on Monday, 16 people were killed and a hospital supported by Doctors Without Borders (MSF) hit by two strikes, in an attack the medical aid group condemned.

"The fact that this attack occurred on a facility while it was treating incoming patients is particularly egregious," said Luis Montiel, MSF's head of mission in northern Syria.