AKCAKALE, Turkey — Tens of thousands of Syrian proxy fighters have been mobilized to take part in a Turkish offensive that appeared imminent on Wednesday against Kurdish forces in Syria, a spokesman said.

Turkey is preparing for an assault on Kurdish forces it brands terrorists after US President Donald Trump announced he was pulling back US troops, despite growing international concern over regional stability.

The Syrian fighters, most of them from northwestern areas controlled by Turkey since previous offensives in 2016 and 2018, were gathered in a former refugee camp in the Turkish border town of Akcakale.

They belong to factions of the Free Syrian Army (FSA), a coalition of groups armed and financed by Ankara.

At least 18,000 fighters are due to participate in the first stage of the Turkish offensive, according to Abdelrahman Ghazi Dadeh, spokesman for Anwar Al-Haq, a small faction within the FSA.

He said 8,000 would target the Syrian border town of Tal Abyad and 10,000 the town of Ras Al-Ain, Dadeh told journalists in Akcakale.

An undetermined number of additional fighters were also expected to be mobilized for an assault on Kobane.

All three towns in northeastern Syria are controlled by the Syrian Kurdish People's Protection Units (YPG).

According to Dadeh, FSA officers accompanied by Turkish soldiers carried out reconnaissance along the border on Wednesday in preparation for the assault.

"There is a 99 percent chance that the offensive will be launched tonight," he said.

Turkey opposes the YPG group due to its ties with the Kurdistan Workers' Party (PKK) which has waged a bloody insurgency against the Turkish state since 1984.

The YPG has been a key ally for the United States in fighting the Islamic State group, but the White House this week announced it was withdrawing special forces from the area that had been effectively preventing a Turkish offensive.

Trump appeared to give a green light to the Turkish operation on Sunday when he ordered the withdrawal of US troops from the border area.

However, he later appeared to alter his position, saying he would "obliterate" the Turkish economy if its forces did anything he considered unreasonable.

Trump has drawn criticism from across the political spectrum for "abandoning the Kurds", who say they lost some 11,000 fighters in battles against IS.

Kurdish authorities in Syria called a general mobilization on Wednesday in anticipation of the Turkish operation. — AFP