KIEV, July 22 - Ukraine's parliament approved a presidential decree on Tuesday to call up more military reserves and men under 50 to fight rebels in eastern Ukraine and defend the border against a concentration of troops in Russia, Reuters reported.

Some 45 days after the latest call-up of additional reserves, which has now expired, Kiev repeated the decree to "declare and conduct partial mobilization" to ensure the ranks of what Ukraine calls its "anti-terrorist operation" are filled.

Ukrainian troops have forced pro-Russian rebels back to their two main strongholds, the cities of Donetsk and Luhansk, slowly taking villages and city suburbs around them.

On Tuesday, heavy fighting went on around the towns of Severodonetsk and Lysychansk near Luhansk and a security official in Kiev said Ukraine's army was aiming to close in on the rebels in Luhansk in the coming days.

The army is under orders not to use air strikes and artillery in the cities, complicating operations to restore control despite Kiev's accusations that the rebels were responsible for the shooting down of a Malaysian airliner. The separatists deny the accusations.

"Russia continues its policy of escalating its armed confrontation," Ukraine's top security official, Andriy Parubiy, told parliament before 232 deputies in the 450-seat parliament voted in favour of the decree.