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 U.S. 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.