AIN ISSA, Syria (Reuters) - The U.S.-backed, Kurdish-led Syrian Democratic Forces (SDF) group said on Monday it was studying whether to send reinforcements to the Afrin region of Syria to help fend off a Turkish attack.

Slideshow ( 2 images )

Turkey launched a cross-border attack on Saturday on the frontier region of Afrin, saying its aim is to crush the YPG, a Syrian Kurdish militia that has been the principal military ally of the United States on the ground against Islamic State.

The YPG militia is the biggest part of the U.S.-backed SDF, which also includes Arab fighters. The YPG said that more fighters would be sent to Afrin if necessary but the area was already well reinforced in anticipation of a Turkish attack.

“We are in the framework of looking at the possibility of sending more military forces to Afrin,” SDF spokesman Kino Gabriel said in a televised news conference, calling for international efforts to halt the Turkish attack.

YPG spokesman Nouri Mahmoud, speaking to Reuters on the sidelines of the news conference, said: “Our forces prepared themselves and were betting on an attack by the current authority in Turkey.”

The SDF and the YPG control a swathe of northern and eastern Syria in addition to the Afrin region in the northwest.

The United States has backed them with air strikes, arms, training and 2,000 troops on the ground, infuriating Turkey which considers the YPG to be allies of Kurdish insurgents that have battled the Turkish state for decades.

Sending any reinforcements to Afrin from SDF-held areas further east would appear to require SDF fighters to pass through territory near Aleppo held by the government of President Bashar al-Assad, complicating the journey in a country where a multi-sided civil war is entering its eighth year.

U.S. special forces do not operate in Afrin. The U.S.-led military coalition has told its Syrian allies that it was working politically to halt the Turkish attack, the SDF spokesman said.

An SDF statement vowed that Afrin would be a “quagmire from which the Turkish army will only exit after suffering great losses” and called on the coalition to meet its responsibility towards “our forces and our people in Afrin”.

The Turkish assault on Afrin opens a new front in a war in which world powers have supported rival sides, while hundreds of thousands of people have died and more than 11 million have been driven from their homes.

Mahmoud, the YPG spokesman, accused Russia -- major military backer of Assad’s government -- of giving permission to Turkey to fly its warplanes over Afrin. Russia withdrew its own forces from the region ahead of the attack. The SDF said Russia had switched off its radars.

President Tayyip Erdogan said on Monday that Turkey had an agreement with Russia regarding its military operation against the YPG in Afrin and that Ankara would not take a step back from the operation.