KIEV, Ukraine (AP) — Army troops have penetrated deep inside a rebel-controlled city in eastern Ukraine in what could prove a breakthrough development in the four-month-long conflict, the Ukrainian government said Sunday.

However, the military acknowledged that another one of its fighter planes was shot down by the separatists, who have been bullish about their ability to continue the battle and have bragged about receiving support from Russia. An Associated Press reporter spotted a column of several dozen heavy vehicles, including tanks and at least one rocket launcher, rolling through rebel-held territory on Sunday.