A United Nations envoy halted his attempts to conduct Syrian peace talks on Wednesday after the army, backed by Russian air strikes, advanced against rebel forces north of Aleppo, choking opposition supply lines from Turkey to the city.

Another senior U.N. official said the Russian escalation was the main reason for the suspension of the peace talks, which have made little progress since beginning earlier this week.

Staffan de Mistura announced a three-week pause in the Geneva talks, the first attempt to negotiate an end to Syria's war in two years, saying they needed immediate help from the rival sides' international backers, principally the United States and Russia.

"I have indicated from the first day that I won't talk for the sake of talking," the envoy, who has described the negotiations as Syria's last hope, told reporters.

A senior U.N. official told Reuters on condition of anonymity that De Mistura halted the talks after Russia's military escalation undermined the negotiating process.

"I think the special envoy decided to suspend the talks because the (United Nations) did not want to be associated with the Russian escalation in Syria, which risks undermining the talks completely," the official said.

"The stepped up air strikes gain the government ground, but also aim at humiliating the opposition on the ground and in Geneva," he added.

Washington and Moscow's support for opposite sides in the five-year-old war, which has drawn in regional states, created millions of refugees and enabled the rise of Islamic State, means a local conflict has become an increasingly fraught global standoff.

De Mistura has said a ceasefire is essential but Russia refused to suspend its air strikes. They helped government forces end a three-and-a-half year siege of the Shi'ite towns of Nubul and al-Zahraa on Wednesday, a step towards recapturing all of Aleppo, Syria's biggest city before the war.

"I don't see why these air strikes should be stopped," Russian Foreign Minister Sergei Lavrov said, saying they were targeting al Qaeda-linked rebels.

Opposition delegation co-ordinator Riad Hijab said there would be no ceasefire until a transition without President Bashar al-Assad was in place.

Moscow accuses Washington, which is backing opponents of Assad, of supporting terrorists, while the US State Department said the air strikes around Aleppo focused mainly on Assad's foes rather than the Islamic State militants Russia says it is trying to defeat.

In a statement on Wednesday, US Secretary of State John Kerry called on the Syrian government and its supporters to halt bombardment of opposition-held areas, especially in Aleppo, and end sieges of civilians in accordance with a U.N. Security Council resolution.

"It is past time for them to meet existing obligations and restore the international community's confidence in their intentions of supporting a peaceful resolution to the Syrian crisis," Kerry added.

The United Nations said it had been told hundreds of families had been uprooted following "an unprecedented frequency" of air strikes in the past two days. Three aid workers were among the dead.

Its envoy had formally opened the peace talks on Friday but both sides denied they had ever begun.

Aleppo rebel factions, reeling from the assault, told the opposition delegation late on Tuesday they would bring down the negotiations within three days unless the offensive ended, a source close to the talks said.

De Mistura halted the talks until Feb. 25 at the latest after meeting the opposition.

"I have concluded frankly that after the first week of preparatory talks there is more work to be done, not only by us but by the stakeholders," de Mistura said.

French Foreign Minister Fabius Laurent Fabius said his government supported De Mistura's decision and he accused Assad and his allies of "torpedoing" the peace effort.

The opposition's Hijab said the pause gave the West a chance to put pressure on the Assad government and Russia to end their assault and that he would not return until there was a change on the ground.

NO END TO RUSSIAN STRIKES

Government delegation chief Bashar al-Ja'afari accused the opposition of pulling out of the talks because it was losing the fight.

Developments on the ground were crucial," he said, accusing de Mistura of providing them with political cover.

"Those who have the responsibility of this failure are the Saudis, Turks and Qataris. They are the real handlers and masters of the Riyadh group."

Aleppo, 50 km (30 miles) south of the Turkish border, was Syria's most populous city before the country's descent into civil war. It has been partitioned into zones of government and insurgent control since 2012.

If the government regains control, it would be a big blow to insurgents' hopes of toppling Assad after a war that has divided Syria between western areas still governed from Damascus and the rest of the country run by a patchwork of rebels.

The Levant Front rebel group said the breaking of the sieges of the Aleppo villages of Nubul and Zahraa came only after more than 500 raids by Russian airplanes.

One commander said opposition-held areas of the divided city were at risk of being encircled entirely by the government and allied militia, and appealed to foreign states that back the rebels to send more weapons.

Diplomats and opposition members said they were taken by surprise when de Mistura called for immediate efforts to begin ceasefire negotiations despite there being no official talks or goodwill measures from the Syrian government.

The opposition has said it will not negotiate unless the government stops bombarding civilian areas, lifts blockades on besieged towns and releases detainees.

The Syrian Observatory for Human Rights, which monitors the war, said Russian and Syrian warplanes launched dozens of strikes on the rebel towns of Hayan and Hreitan in northern Aleppo on Wednesday.

"Less than 3 km separate the regime from cutting all routes to opposition-held Aleppo," Observatory director Rami Abdulrahman said. "It did in three days what it failed to do in 3-1/2 years." – Reuters