Both the government troops and the separatists agreed during the weekend to pull their big guns 25 to 70 kilometers (15 to 43 miles) back, depending on the type of the weapon, thus creating a buffer zone. However, Ukrainian military spokesman Anatoliy Stelmakh told reporters on Monday that the move will not start before rebel attacks stop completely, in accordance with the conditions of the truce.

Stelmakh said there were two artillery attacks overnight and although this is significantly fewer than in previous days, "as long as firing on Ukrainian military positions continues, it's not possible to talk about a pullback."

Kyiv is only willing to start pulling back its heavy weapons once there has been a total cessation of firing for a whole day, according to army sources.

Kharkiv mourns victims of terror strike

The shaky peace deal was put under additional strain after a terror strike in Ukrainian city of Kharkiv on Sunday, which killed two people and injured 15 more as pro-Western Ukrainians marked one-year anniversary of the Maidan revolution. One of the victims was a local police official and the other a Maidan movement activist, according to the sources of the Interfax news agency.

A spokesman for the national security service, Markian Lubkivskiy, said four people have been arrested for the bombing. The suspects were detained while carrying a portable rocket launcher in the car, and appeared to acting under instructions from Russia.

"We can clearly see that the launcher was received from the (Russian city of) Belgorod. Instructions were received from the Russian Federation," he said in broadcast remarks.

Kharkiv is well away from the frontline and has so far remained relatively unaffected by clashes. There have been several bombings in Kharkiv and other majority-Russian-speaking cities since the conflict began, but they usually only result in damage to property.

Head of the Kharkiv regional state administration Igor Rainin said that the Sunday attack was meant to inspire fear.

"I view this cynical and brash strike as an attempt to scare the citizens of Kharkiv and to widen the zone threatened by terrorists," he said in a statement.

Two of the injured remain in critical condition.

Tusk in Kyiv

On Monday, Kyiv also accused the rebels of attacking their positions near port city of Mariupol, driving the tensions even further. Kyiv claims Russia had sent 20 tanks and additional heavy weapons to near Mariupol. Moscow denies supporting the rebels.

Rebel seizure of Mariupol could help toward establishing a land corridor between mainland Russia and Crimea, which is controlled by Russia.

President of the European Council Donald Tusk, who participated in a Kyiv rally marking one year since Maidan revolution on Sunday, said he would "begin consultations on Monday to increase some of the measures in connection with the aggression" in Ukraine, the Interfax-Ukraine news agency reported.

dj/rc (AP, AFP, Reuters, Interfax, dpa)