BEIRUT — Syrian government forces and allied militias, backed by what an aid official said were “hugely increased” Russian air strikes, pushed ahead with a major offensive in northern Syria on Friday, moving a step closer to encircling contested Aleppo, Syria’s largest city. In recent days, pro-government forces have captured several towns and villages north of Aleppo, driving a deep wedge into rebel-held areas and cutting off a rebel supply road to Turkey. Air strikes that mainly target opposition forces are “undermining efforts to find a political solution to the conflict,” NATO Secretary-General Jens Stoltenberg said Friday. Earlier this week, a U.N.-led attempt to launch indirect talks between a government delegation and opposition representatives in Geneva was adjourned after several days of acrimonious bickering. In Friday’s fighting, pro-government troops retook the village of Ratyan north of Aleppo, one of several communities they captured in the area this week, according to the state news agency SANA and the Britain-based Syrian Observatory for Human Rights.