As many as 600 Islamic State (Isis) fighters have been killed in Syria over the past three weeks, losing thousands of square kilometres in the process, according to the American secretary of state.
Meanwhile a truce between the Syrian authorities and the rebels has cut violence by 80 to 90%, a fact John Kerry labelled "very, very significant".
Kerry made the announcement at a meeting of the coalition of nations allied against Isis (IS) in Paris.
"In Syria, over the last three weeks alone, Daesh [IS] has lost 3,000 sq km [1,160 sq miles] and 600 fighters," he said.
The truce, which has not been ratified by jihadist groups like IS or the Nusra Front, took effect late last month. Fresh peace talks between the Syrian government and its official opposition are set to proceed in Geneva in Switzerland tomorrow, on the fifth anniversary of a war which is believed to have claimed more than a quarter of a million lives.
"We look forward to the resumption of talks in Geneva on Monday," said Kerry, in a joint news conference with senior European diplomats also involved in the UN-brokered peace talks.
Kerry reserved some criticism for the Syrian Foreign Minister Walid al-Moallem, who he said had "clearly tried to disrupt the process" of peace negotiations, by claiming that the Syrian government delegation would refuse to stay for more than 24 hours if the opposition doesn't show up - and that that the removal of President Bashar al Assad would cross a "red line".
The Syrian government has so far resisted requests to hold democratic elections in a country which has been ruled with an iron fist by the al-Assad dynasty since 1971. The UN is quietly pressing for elections within 18 months, which is sure to be one of the key issues in the upcoming talks.
Kerry himself said "the fact is that [his] stronger sponsors, Iran and Russia, have both adopted ... an approach which dictates that there must be a political transition and that we must move towards a presidential election at some point in time".