BEIRUT — Syrian government forces pushed into one of the last remaining urban strongholds of the Islamic State in the country’s east Friday, after days of fierce fighting and intense Russian air strikes that involved cruise missiles from the Mediterranean, activists and officials said. The push into the town of Mayadeen came as al Qaeda-linked fighters attacked a key central Syrian village at the crossroads between areas under government control and those controlled by insurgents, activists said. Taking Mayadeen would mark another blow to the extremist group, which has lost wide areas of Iraq and Syria in its self-declared caliphate over the past year.