A border guard vessel has been attacked by land-based forces, Ukraine said, hours after Russian president Vladimir Putin called for immediate talks on a political solution to the crisis in the east of the country.

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 today.

Separatists have shot down a Ukrainian fighter plane amid fighting between pro-Russian rebels and Ukrainian government forces in the eastern part of the country.

Ukrainian troops have taken control of a key checkpoint on the western edge of Donetsk, the biggest city in the rebellious east, and are making gradual advances to quash pro-Russian separatist forces.

Two Ukrainian military fighter jets have been shot down in the east, according to the country’s Defence Ministry.