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 on Monday.
The Britain-based Syrian Observatory for Human Rights said among those killed were 50 foreign fighters.
On May 31, the Kurdish-led Democratic Forces of Syria (DFS), backed by U.S.-led airstrikes, launched a wide scale 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 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.
On Sunday, regime forces and allied militiamen advanced to within less than 7 km of Tabqa’s military airport, the observatory said.
The DFS has 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 has 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.