Syrian regime forces advanced against rebels during intense street battles in the heart of Aleppo today, after the United States abandoned talks with Russia aimed at reviving a ceasefire deal. The UN rights chief appealed for action to halt the ghastly avalanche of violence unfolding in Syria's second city, which is reeling from some of the most brutal fighting in the five-year conflict. <br/><br/>The Syrian army announced a major Russian-backed military push nearly two weeks ago to capture the rebel-held eastern half of Aleppo. Today, loyalist fighters seized several high-rise buildings from rebel groups in the city centre, pushing north towards other opposition districts.<br/><br/>The Syrian Observatory for Human Rights, a British-based monitor, said the regime forces are gradually advancing after street battles on the front line dividing the rebel-held east from the government-controlled west. Syrian state news agency SANA reported that rebel shelling on the government-held west, including on the Aleppo University campus, left six people dead today. <br/><br/>The United States and Russia back opposing sides in the conflict. Washington announced late yesterday that it would suspend joint efforts to reinstate a nationwide truce, accusing Moscow of abetting strongman Bashar al-Assad's assault on Aleppo.