Syria's Foreign Minister Walid Muallem says the next target after the liberation of Idlib is to retake the area east of the Euphrates, which is held by US-backed Kurdish militants.

"After Idlib our target is east of the Euphrates," he said at a press conference with his Iraqi counterpart Ibrahim al-Jaafari in Damascus Monday, stressing his country’s determination to restore sovereignty over all Syrian territories.

Russian Foreign Minister Sergei Lavrov said on Friday the US is seeking to establish a quasi-state east of the Euphrates River and create illegal structures there.

“On Syrian territory there are vast lands east of the Euphrates where absolutely unacceptable things are taking place,” Lavrov told France's Le Figaro.

“The US is trying to use these lands through their Syrian allies – above all, through the Kurds – in order to establish a quasi-state there,” he went on to say.

The US has been conducting airstrikes against what it says are Daesh positions inside Syria since September 2014 without any authorization from the Damascus government or a United Nations mandate.

The US and its allies have repeatedly been accused of targeting and killing civilians and drawn criticism over their failure to destroy Daesh, which they claim to be fighting.

Muallem on Monday said the US has destroyed Raqqah Province under the pretext of fighting Daesh. US-sponsored militants are refusing to hand back Raqqah to the government control after taking it over from Daesh last year.

Amnesty International’s new Secretary General Kumi Naidoo said on Friday he had been shocked by the horrific destruction and utter human devastation left by US airstrikes in Raqqah.

Muallem said Washington continues to support Daesh and evacuate its members to the east of the Euphrates.

A retaliatory attack by Iran's Islamic Revolution Guards Corps late last month hit Daesh headquarters near Albu Kamal, east of the Euphrates, with a US defense official telling CNN that Iranian missiles came within three miles of US troops in the country.

In his remarks, Muallem touched on a deadline which passed Monday without Takfiri terrorists leaving a planned buffer zone in Idlib as set out under a Russian-Turkish deal.

The minister said other options are on the table if the militants do not comply with the requirements of the deal, without elaborating.

The deal, agreed last month on Turkey's initiative, forestalled an imminent offensive by the Syrian government forces to liberate the province, the militants’ last major stronghold in Syria.

"We cannot keep quiet about the continuation of the current situation in Idlib if the Nusra Front refuses to comply with this agreement," Muallem said.

The minister said it was up to Moscow now to judge whether the deal had been fulfilled.

"Now we must give the time to our friends the Russians to judge whether the agreement was fulfilled or not," he said.

Muallem said Ankara has the means to ensure the agreement is complied with.

"For us we said always Idlib, as any other province, has to return back to Syrian sovereignty. We prefer to have it through peaceful means, through reconciliation, but if not there (are) other options," he added.