KIEV, Feb. 15 – Ukraine's military said on Sunday pro-Russian rebels had shelled Ukrainian positions on 10 separate occasions since a ceasefire came into effect at midnight.

"As of the morning of February 15, there have been 10 shelling attacks on our positions by rebels - mainly in the area of Debaltseve," spokesman Anatoly Stelmach said by telephone, Reuters reported.

A Reuters witness in east Ukraine heard the sound of heavy artillery coming from the direction of Debaltseve, a key transport hub where government forces have been hard pressed by encircling separatists.

The shelling comes as rebels disavowed a new truce on Sunday hours after it took effect, saying it did not apply to the town where most fighting has taken place in recent weeks.

Guns fell abruptly silent at midnight across much of eastern Ukraine in line with the ceasefire agreement, reached after a week of diplomacy led by France and Germany.

But pro-Russian rebels announced they would not observe the truce at Debaltseve, where Ukrainian army forces were encircled and Kiev military said rebel attacks on the town steadily increased from mid-afternoon on Sunday.

"Of course we can open fire (on Debaltseve). It is our territory," senior rebel commander Eduard Basurin told Reuters. "The territory is internal: ours. And internal is internal. But along the line of confrontation there is no shooting."

A statement by the Kiev military on Sunday night said the "enemy" was carrying out attacks with varied types of weapons, including Grad rocket systems, and had a plan to try to seize Debaltseve from the west.

In a four-way telephone conversation with the leaders of Germany, France and Russia's Vladimir Putin, President Petro Poroshenko said the position of the four at peace talks last week in Belarus had been for a ceasefire on all the front lines including at Debaltseve.

Poroshenko stressed that a withdrawal of military equipment and heavy weapons required a "full and unconditional" ceasefire under the Minsk agreement, his press service said.

The Organization for Security and Cooperation in Europe, responsible for monitoring the ceasefire, said rebels had denied its observers access to Debaltseve.

Both sides blamed what firing there was on the enemy. But Debaltseve has been the focus of fighting for weeks, and it will be hard to speak of a truce if Ukrainian troops remain trapped there under fire, or the rebels press on with their advance.

Kremlin spokesman Dmitry Peskov said Thursday's peace deal including the truce must be implemented "unconditionally". But he made no mention of whether Moscow believed the truce applied to Debaltseve.

Ukrainian forces have been holding out for weeks in the town, which sits astride a railway junction in a pocket between the two main rebel strongholds.

Rebels say they have completely encircled the town, but Ukraine says its forces have kept open a road to resupply it in the face of a Russian-backed onslaught.

Washington says regular Russian forces armed with tanks and missile launchers advanced on the town from all sides in the days before the truce.

Reuters journalists operating on the rebel side have seen armored columns of troops without insignia arriving in the area in recent days.

In the main rebel center, Donetsk, Reuters journalists said artillery had been exploding every few seconds in the hours before the ceasefire, but halted abruptly at midnight.

A Ukrainian military spokesman said the ceasefire was being "generally observed". Its forces had been shelled 10 times since the truce took effect in localized incidents, and no soldiers had been killed.

Poroshenko, wearing the uniform of the armed forces' supreme commander, announced the order to stop firing in a midnight televised address, but said there was still alarm over Debaltseve.

"I very much hope that the last chance to begin the long and difficult peaceful process for a political settlement will not be wasted," he said, adding, however, that if Ukraine were slapped, it would not "turn the other cheek".

The ceasefire, negotiated in all-night four-power talks on Thursday, foresees creation of a buffer zone and withdrawal of heavy weapons. More than 5,000 people have been killed in a conflict that has caused the worst crisis in Russia-West relations since the Cold War. (rt/ez)