Partial conscription will resume and authorities want 100,000 reservists mobilised to strengthen the frontline as violence escalates after a period of relative calm.
“Russian aggression is continuing. There has been a significant surge in the intensity of firing,” Oleksander Turchynov, secretary of the national defence council, told parliament, adding that 8,500 Russian regular forces 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.
A shell or rocket attack on a passenger bus at an army checkpoint on Tuesday left 13 civilians dead.
Kiev blamed pro-Russian rebels.
They deny responsibility.
Seeking answers, the OSCE European security watchdog will cooperate with Kiev and Moscow – members of the joint working group the JCCC – as well as members of the self-proclaimed ‘Donetsk People’s Republic’.
“The Ukrainian representative to the JCCC, the representative of the Russian Federation to the JCCC and the DPR members will form a joint investigation into that incident,” said OSCE Deputy Chief Monitor in Ukraine, Alexander Hug.
Almost 5,000 people have been killed in the conflict between Ukrainian government forces and pro-Russian rebels.
Moscow denies it has any troops in the east of Ukraine.
Victims of the bus attack were being honoured on Thursday with a day of mourning decreed by Ukrainian President Petro Poroshenko.