KIEV, Jan. 15 - Fighting intensified around the Donetsk international airport on Thursday as pro-Russian separatists stepped up efforts to dislodge government forces and Ukraine's military said two of its soldiers had been killed, Reuters reported.

The complex, a battered wreck with its runways cratered by shell explosions, has not functioned as an airport since hostilities erupted last April but it remains an important symbol for both sides.

"Unfortunately, there are two (Ukrainian soldiers) killed," Vladyslav Seleznyov, a senior member of the Ukrainian army general staff, told Interfax news agency.

In Kiev, parliament voted to refresh its front-line forces and resume partial conscription after a top security official warned that Russian forces backing separatist rebels had sharply increased military activity in the east of the country.

"Russian aggression is continuing. There has been a significant surge in the intensity of firing," Oleksandr Turchynov, secretary of the national defense council, told parliament, adding that 8,500 Russian regular servicemen were now deployed in eastern Ukraine.

Two Ukrainian soldiers were killed and four wounded on Wednesday when Ukrainian positions were fired on 129 times, which Turchynov said was a record for this year so far.

Despite what the West and Kiev say is incontrovertible evidence, Moscow denies its troops are involved in the conflict in which more than 4,700 people have been killed.

The warning of increased military activity by Russian forces followed the shelling of a passenger bus on Tuesday at an army checkpoint. Twelve civilians were killed.

Candles were lit in public buildings on Thursday on a day of mourning for the victims. Kiev has blamed the separatists for the attack but they have denied responsibility.

Ukraine's parliament supported a decree of President Petro Poroshenko to swap out long-serving troops at the front and to bring in veterans from the reserve as well as resuming partial conscription.

Ukraine scrapped compulsory military call-up in 2013 before the ousting of a pro-Moscow president, Viktor Yanukovych, which sparked the confrontation with Russia.

"There is an urgent need to strengthen the combat and mobilization readiness of our forces and other military forces up to a level which guarantees an adequate reaction to threats to national security from continuing Russian aggression," Turchynov said. (rt/ez)