Turkey says it has launched a much-awaited air and ground offensive against the Kurdish-controlled enclave of Afrin in northern Syria.

After days of shelling, Turkish fighter jets on Saturday carried out air raids on the border district targeting positions held by the Syrian Kurdish PYD and YPG groups.

The heavy bombardment began as units of pro-Ankara rebels known as the Free Syrian Army (FSA) started moving into Afrin, according to the state-run Anadolu news agency.

Turkish President Recep Tayyip Erdogan said on Saturday that the operation in Afrin would be followed by a push in the northern town of Manbij, which the US-backed Kurdish forces captured from ISIL in 2016.

Turkey considers Syria's Kurdish Democratic Union Party (PYD) and its armed wing, the YPG, "terrorist groups" with ties to the banned Kurdistan Workers' Party (PKK), which has waged a decades-long fight inside Turkey.

The US has previously armed the YPG, viewing it as the most effective ground force in its fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group.

Erdogan said that all Kurdish armed groups "are all the same" and that changing their names "does not change the fact that they are terror organisations".

According to estimates, there are between 8,000 to 10,000 Kurdish fighters in the Afrin area.

'Complicated situation'

Al Jazeera's Stefanie Dekker, reporting from Antakya in Turkey, said the launch of the operation followed a week of "increasingly strong political rhetoric" coming from Turkish officials.

"The Turkish army says it is only targeting what it calls 'terrorists' ... and not civilians - but certainly it will be terrifying for civilians in that area because they are surrounded," she added.

"To highlight the complexity of this war, there is now a NATO ally, Turkey, bombing a group that the US calls its best ally when it was fighting ISIL on the ground and still continues to do so - so it's an incredibly complicated situation."

In recent days, Ankara has been repeatedly threatening to crush the Syrian Kurdish fighters.

On Friday, Turkey's Defence Minister Nurettin Canikli said his country would go ahead with its military offensive in Afrin, saying Syrian Kurdish fighters there pose a "real" threat to his country.

Ankara fears the establishment of a Kurdish corridor along its border and had been deploying troops and tanks there in preparation for the ground assault.

"We will wipe out this corridor step-by-step, starting from the west," Erdogan said on Saturday. "Afrin operation has de facto started in the field. This will be followed by Manbij."

Syria had earlier warned against any operation and said it would shoot down Turkish fighter jets.

Ground push

On Friday, Turkey mobilised thousands of FSA rebels to Hatay province near the Syrian border, as part of the planned offensive.

Anadolu reported that the FSA rebels were taken "under extensive security" in a convoy of at least 20 buses, from the province of Kilis.

Last year, Turkey launched Operation Euphrates Shield, in which Turkish-backed FSA rebels cleared a large part of northern Syria of armed fighters.

"The language coming from the Turks has been that the Afrin operation is going to be the start, then they are going to move into Manbij and then all the way to the Iraqi border," said Dekker.

"Manbij is a town west of the Euphrates, the YPG remains there and Turkey always wanted the YPG to move east of the Euphrates. The last time there was a confrontation there between the two sides the Americans moved in with troops and vehicles to calm that down."

Following the start of Turkey's air campaign, the defence ministry of Russia, which controls the airspace over Afrin, said it was pulling back soldiers that had been deployed near the city.

It said in a statement that "to prevent possible provocations, to exclude the threat to life and health of Russian servicemen, the operational group of the Center for Reconciliation of warring parties and military police in the Afrin area is relocated to the Tell-Adjar area".

Moscow's military intervention in Syria in 2015 turned the war in favour of Syrian President Bashar al-Assad.

"It's now clear that the Russians have given tacit green light to this [Turkish operation] because they control the airspace - so seeing Turkey flying air sorties means the Russians have agreed to this."

"However, we have also heard from the Russians that they are concerned that they are watching it closely and that they urge constraint."

This came as Turkish officials reportedly had discussions with US and Russian officials, as well as a rare official contact with Assad's government.

Meanwhile, Stephane Dujarric, spokesman of UN Secretary-General Antonio Guterres, warned against more military activities in Syria.

"We've seen the reports of shelling in Afrin. We reiterate our call on all concerned parties to avoid further escalation and any acts that could deepen the suffering of the Syrian people," Dujarric said.

"All parties must ensure protection of civilians at all times, under any circumstances."

The US had also urged Turkey to avoid taking action against the Kurdish rebels, urging Ankara to keep its focus on ISIL.

There have been reports that the US was also trying to recruit Kurdish fighters in Syria to fight against ISIL.

In response, Turkey warned that its relations with the US would be "irreversibly harmed" if Washington moves to form the 30,000-strong army in the north of Syria.