Some 450 Islamic State fighters have been killed since Kurdish-led forces launched an offensive against the city of Minbij in northern Syria, a monitoring group said Monday.
The Britain-based Syrian Observatory for Human Rights said among the killed were 50 foreign fighters.
On May 31, the Kurdish-led Democratic Forces of Syria (DFS), backed by US-led airstrikes, launched a widescale offensive on the Minbij pocket, the extremists' last territory on the Turkish border.
The operation is aimed at cutting the extremist group's last link with the outside world and isolating its de facto Syrian capital, al-Raqqa, which lies south-east of Minbij.
Meanwhile, the observatory said that the Islamic State movement has sent some 300 fighters to bolster its forces against a regime offensive near the city of Tabqa, just west of al-Raqqa.
The jihadists managed to regain ground from the regime forces, pushing them back to 15 kilometres from Tabqa airport after they reached within 7 kilometres of it on Sunday, the observatory said.
The DFS have also launched an operation north of al-Raqqa, with part of their forces pushing towards Tabqa, but their main focus appears to be on Minbij.
Islamic State has lost ground on several fronts in recent weeks. The DSF have surrounded Minbij, while Iraqi forces have pushed into the centre of Fallujah, its stronghold west of Baghdad.
Its Libyan branch has also lost most of its territory recently, with forces loyal to a UN-backed national unity government closing in on its main stronghold of Sirte.
However, the group retains its core territory in eastern Syria and northern Iraq.