Hundreds of Kurdish fighters have arrived in eastern Syria to help a US-backed alliance fight the Islamic State (IS) militants after a major setback last week, a monitor said Tuesday.
The Syrian Democratic Forces (SDF), a Kurdish-Arab alliance backed by the US-led coalition, launched an offensive on September 10 to expel IS from their holdout of Hajin on the Iraqi border.
They advanced slowly with support from coalition air strikes, but faced sand storms and a vicious fightback including suicide bombers, which forced them to retreat on Sunday.
According to the Syrian Observatory for Human Rights, a Britain-based monitor, at least 72 SDF fighters were killed in last week's IS counter-attack, one of the jihadist group's deadliest operations this year.
The monitoring group said hundreds of Kurdish fighters, men and women, had arrived on the outskirts of the Hajin pocket since then.
"Since Sunday, over two days, 500 fighters from the Kurdish special forces, the People's Protection Units and the Women's Protection Units have been sent," Observatory chief Rami Abdel Rahman said.
Later on Tuesday, he said "at least 100 SDF fighters" had arrived from the northern city of Manbij, bringing the total number of reinforcements to arrive over the past two days to 600.
Several thousand SDF fighters were already present in the area, he said.
An SDF spokesman said Kurdish fighters "experienced in fighting IS" had been sent as reinforcements to the Hajin front, but said he could not confirm numbers.
"These units will take part in fighting IS on the Hajin front," Mustefa Bali said.
On Sunday, an SDF commander told AFP that military reinforcements and heavy weapons had been sent to the front.
He said the alliance would launch a new assault as soon as the reinforcements had arrived.
Coalition strikes on the area Tuesday killed at least nine jihadists, the Observatory said.
More than 300 SDF fighters and around 500 IS jihadists have been killed in the past seven weeks of fighting, according to the monitor.
The coalition estimates that 2,000 IS fighters remain in the Hajin area.
IS overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" across territory it controlled.
But the jihadist group has since lost most of that territory to various offensives in both countries.
In Syria, its presence has been reduced to parts of the vast Badia desert and the Hajin pocket.
More than 360,000 people have been killed and millions displaced since Syria's war erupted in 2011 with the brutal repression of anti-government protests.