Islamic State has lost 600 fighters and thousands of square kilometres over the past three weeks, the US Secretary of State says.

John Kerry made the comments after talks with European allies in Paris on the Syrian conflict.

"In Syria, over the last three weeks alone, Daesh has lost 3,000 sq km (1,160 sq miles) and 600 fighters," he said, using another name for Islamic State.

Mr Kerry also said a recent truce has reduced violence in the country by 80-90%, which he said was "very, very significant".

The landmark truce signed between the Syrian regime and rebels - but not by jihadist groups like Islamic State and Nusra Front - took effect late last month.

Fresh peace talks between the Syrian regime and opposition are due to start on Monday in Geneva - at the start of a week marking the fifth anniversary of a conflict that has killed more than 250,000 people, allowed for the expansion of Islamic State and created a growing refugee crisis.

On the eve of the UN-brokered talks, Mr Kerry criticised remarks made by Syrian foreign minister Walid Muallem that removing President Bashar al Assad would cross a "red line".

Mr Kerry said his comments were "clearly trying to disrupt the (peace) process... (he was) clearly trying to send a message of deterrence to others".

Mr Assad's future has long been a major stumbling block. Syria's key ally Russia rejects any suggestion he should go, while opposition-backer the US is calling for him to step down.

French foreign minister Jean-Marc Ayrault said Mr Muallem's comments were "a provocation and a bad sign and doesn't correspond to the spirit of the ceasefire".

The Syrian opposition, meanwhile, says Mr Assad must go - dead or alive.

"We believe that the transitional period should start with the fall, or death, of Bashar al Assad," chief opposition negotiator Mohammad Alloush said.

Meanwhile, Islamist militants have launched a successful offensive against Western-backed rebels in Syria, according to Syrian Observatory for Human Rights.

Jihadists with Nusra Front and Jund al Aqsa seized bases and weapons from Syrian Free Army faction Division 13 in Idlib province during clashes that left six people dead, the British-based human rights group.