Isis has lost 600 fighters and thousands of square kilometres of territory over the past three weeks, according to the US Secretary of State.
John Kerry made the revelations in Paris after holding talks on the Syrian conflict with the foreign ministers from France, Britain, Italy and Germany.
"In Syria, over the last three weeks alone, Daesh has lost 3,000 sq km (1,160 sq miles) and 600 fighters," said Mr Kerry, using an alternative term for Isis employed by Western leaders to undermine the radical Islamic group’s claims to statehood.
Greater London measures around 1,572 sq km.
Mr Kerry added that the recent ceasefire in Syria has reduced violence in the war-torn state by 80-90 per cent, which he described as a "very, very significant" development. The landmark truce signed between the Syrian regime and rebels – but notably not by jihadist groups such as Isis and Nusra Front – began last month. More than 270,000 people have been killed since the civil war broke out in March 2011, according to the Syrian Observatory for Human Rights.
The Syrian government and opposition are due to begin fresh peace talks in Geneva on Monday.
Ahead of the UN-brokered talks, Mr Kerry criticised Syrian foreign minister Walid Muallem for saying that removing President Bashar al Assad would cross a "red line". During a televised news conference in Damascus, Mr Muallem said: “We will not talk to anyone who talks about the position of the presidency. Bashar [al-Assad] is a red line; the property of the Syrian people.”
Mr Kerry responded by saying his comments were "clearly trying to disrupt the [peace] process... and [that he was was] clearly trying to send a message of deterrence to others."
He continued: "But the fact is that 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."
Jean-Marc Ayrault, the French foreign minister, said Mr Muallem's comments were "a provocation and a bad sign and doesn't correspond to the spirit of the ceasefire".
Russia, a key ally of the Syrian regime, have stood firm in their support or Mr Assad, refusing to back any agreement that would see him leave his position. The Syrian government echoed this on Saturday, saying it would not entertain the idea of discussing presidential elections at the Geneva talks.
Mr Kerry is in the midst of determining whether to formally declare that Isis’ atrocities against religious minorities, against groups including Christians, Yezidis and Kurds, constitute "genocide, war crimes and crimes against humanity."