By Pavel Polityuk and Maria Tsvetkova
KIEV/DONETSK, Ukraine (Reuters) - Three Ukrainian servicemen were killed and nine wounded as pro-Russian rebels shelled government positions despite a ceasefire deal, the military said on Tuesday, announcing Kiev's highest casualty toll in several days.
The losses underscore the fragility of a two-week-old ceasefire agreement which Ukrainian President Petro Poroshenko has called the last chance for peace between Kiev and the separatists he says are being armed by Russia.
After the separatists initially ignored the ceasefire to launch one of the biggest offensives of the war, fighting finally slowed down dramatically last week. In recent days both sides have been withdrawing artillery from the front line, the next stage in a peace agreement brokered by France and Germany.
Both sides still accuse each other on a daily basis of breaking the truce, however, and Tuesday's shelling appeared more intense than in previous days.
In rebel-held Donetsk, among the ruins of the city's airport, artillery fire was heard coming towards rebel positions from the direction of Ukraine's frontline stronghold of Pisky. "We have no mortar rockets anywhere here. We have already withdrawn everything bigger than 100 millimetres in accordance with the Minsk agreements," said the Donetsk rebels' self-styled defence minister Vladimir Kononov. "If the Ukrainians continue to shoot...you just heard incoming fire...then I think it will lead to an end of the ceasefire," he said among the loud explosions. In Kiev, a military spokesman said rebels had shelled Ukrainian positions 22 times over the past day.
"An intensification of the enemy's military operations was observed on the evening of March 2. They shot at Pisky with mortar bombs and with an anti-aircraft system at Avdiyivka," said Ukrainian military spokesman Anatoly Stelmakh, naming two towns that have suffered heavy damage in the conflict.
Ties between Russia and the West have plummeted to Cold War-era lows over the violence, and Washington and Brussels accuse Moscow of arming the separatists and reinforcing their ranks with Russian troops.
Russia has repeatedly denied involvement and blames the United States for pushing the pro-Western government in Kiev into armed conflict.
Some in Kiev have voiced worries that the rebels are using the ceasefire to prepare for more attacks around the strategically important Ukrainian-held port city of Mariupol in southern Donetsk province.
"The enemy is continuing to regroup and there is a significant accumulation of forces in the south," said Interior Ministry adviser Zoryan Shkiryak.
Rebels deny planning an attack on Mariupol, saying they want to take control of the port through negotiations.
Continued violence, including the rebels' storming of a strategic transport hub in Debaltseve on the fourth day after the ceasefire had been due to take effect, slowed implementation of the peace deal reached in the Belarus capital Minsk.
The agreement outlines the ceasefire and withdrawal of heavy artillery, and calls for progress to be monitored by the OSCE European rights and security watchdog.
Journalists have seen guns being sent away from the front line on both sides, but the OSCE says it cannot confirm the withdrawal of weapons as its monitors have not been given access to see where some of the guns have been relocated.
INCOMING FIRE
The West has pinned hopes on the Minsk agreement, and NATO Secretary-General Jens Stoltenberg told a news conference in Brussels that while "incidents" remained, the overall picture showed that the ceasefire was holding.
"It is important that all sides respect their commitments and that the separatists backed by Russia do not use this pause in the fighting to prepare for a new offensive," he said.
"All heavy weapons have to be moved from the frontline in accordance with the Minsk agreements and the OSCE monitors have to have full access to the area to be able to monitor the ceasefire."
The leaders of Germany, Ukraine, France and Germany said on Monday they agreed that the OSCE needed a broader role as observers of the ceasefire agreement and the removal of weapons.
Poroshenko in a meeting with security chiefs said Kiev must cooperate more with OSCE monitors, who should be deployed around areas where the ceasefire has been repeatedly violated.
Ukraine's parliament is expected this week to approve a request from Poroshenko for international peacekeepers to monitor the conflict, an idea that has received a lukewarm reception in European capitals and is scorned by Moscow.
(Additional reporting by Natalia Zinets and Thomas Grove in Kiev and Adrian Croft in Brussels; Writing by Thomas Grove; Editing by Peter Graff)