The chief peace negotiator of Syria's mainstream opposition said on Sunday he was resigning over the failure of the U.N.- backed Geneva peace talks to bring a political settlement and to ease the plight of Syrians living in besieged opposition-held areas.

Mohammed Alloush, who is also the representative of the powerful Jaish al Islam opposition faction in the Saudi-based High Negotiations Committee (HNC), said in a statement sent to Reuters that the peace talks had also failed to secure the release of thousands of detainees or to push Syria towards a political transition without President Bashar al Assad.

The U.N.-backed parties have not set a date for the resumption of the peace talks after the HNC suspended their participation until the situation on the ground has radically changed.

Alloush also said that without any of the opposition demands met, peace talks were a "waste of time", adding that he did not expect peace talks to resume so long as the Assad regime remained intransigent and not ready to enter "serious negotiations".

The regime does not recognise the right of the HNC to speak on behalf of the opposition and insists they were tools of foreign powers seeking to topple Assad and brand Alloush himself as a "terrorist".

The resignation was accepted in a meeting in the Saudi-capital Riyadh headed by HNC's chief coordinator Riad Hijab that sought to assess the peace negotiations.

Separately, the Turkish based Syrian opposition affiliated to the HNC called on foreign backers to step up military support for the moderate Free Syria Army (FSA) opposition groups.

They said such backing would allow their fighters to wrest back the mainly Arab inhabited city of Raqqa, the defacto capital of DAESH in Syria.

The opposition criticised the arming and training of the PKK affiliated U.S. backed Syria Democratic Forces (SDF), whose main component are the YPG militia for pursuing a separatist agenda.

With the help of U.S special forces, they launched last week with allied Arab tribal groups an assault north of the city of Raqqa with the aim of capturing it.. They had gained a string of villages around Ain Issa, a town about 60 km north west of Raqqa city.

Head of the main Syrian opposition delegation Asaad al-Zoubi also told al Hadath TV channel that he too wanted to be relieved of his post in the HNC but did not confirm he had taken a similar step.

A source in the opposition said Zoubi was replaced in a reshuffle of the HNC negotiating team that includes both military factions and political groupings.

Zoubi said no real peace talks had taken place four months since the latest rounds of Geneva peace talks were launched and opposition pleas to get the U.N.-backed process to pressure the regime authorities to allow humanitarian aid to besieged areas went unheeded.

The Syrian opposition suspended in April its formal participation in peace talks in protest at Assad regime offensives they said meant a ceasefire was effectively over. They were however under strong pressure not to quit altogether.

FSA leader: Obama stabbed us in the back

The Chief of Staff of the Free Syrian Army (FSA), Ahmed Berri, speaking to the Yeni Şafak daily on Sunday criticized what he called the biased stance of the United States on the FSA. Berri gave details of the current situation in northern Aleppo and gave further information about the military operations in Raqqa. He said: "We have lost 500,000 people due to [the Bashar] Assad regime along with the alliance with Iran and Russia. Obama administration, which kept silent over 20 million Syrians, had stabbed us in the back with its actions."

He said the U.S. is unconcerned with the humanitarian plight taking place in Syria. Describing the Democratic Union Party (PYD) as a Marxist-oriented separatist group, Berri said: "The U.S. mobilized all of its resources in order to help the PYD." In an interview, U.S. President Barack Obama spoke about the U.S. alliance with the PYD: "Oftentimes, the challenge is if you have former farmers or teachers or pharmacists who now are taking up opposition against a battle-hardened regime, with support from external actors that have a lot at stake, how quickly can you get them trained; how effective are you able to mobilize them."

Berri also commented on what he called the U.S.'s inconsistent attitude, and said: "Syrian citizens are losing hope for the future. Meanwhile, the U.S. claims it they is peaceful, but there is a tendency to establish a PYD government [in northern Syria]." According to Berri, with U.S. involvement, hostility toward Sunnis has become a common ground for all other groups to unite. He said that DAESH did not strike back to the PYD's People's Protection Units- (YPG) dominated Syrian Democratic Forces (SDF) movement toward Raqqa, and claimed: "DAESH and the PYD are being controlled by the same superior mind."

More than 160,000 civilians have been trapped by the fighting between DAESH and Syrian opposition and the aid group Doctors Without Borders last week evacuated one of the few remaining hospitals from the Aleppo area.

Meanwhile, the International Rescue Committee (IRC) said that over the weekend, more than 8,000 people managed to escape villages and displacement camps to the east and south of the opposition-held town of Azaz.

IRC said that before the road, became too dangerous, some 6,000 people managed to flee the rebel stronghold of Marea to seek safety in Azaz. It added that more than 1,000 people managed to reach Afrin and more than 1,200 people have fled to a makeshift refugee camp on Yazibag mountain.