Kiev: Ukraine on Thursday renewed accusations of a Russian military build up on its border and approved fresh troop mobilisations as a wave of violence threatened all-out conflict in the country`s war-torn east.
A national day of mourning was held for 13 people killed on Tuesday when a rocket exploded near a commuter bus travelling towards the Ukrainian city of Donetsk, the worst loss of civilian life since a September truce that only partially halted the violence.
An upsurge in fighting has rocked eastern Ukraine in recent days, with a planned peace summit between President Petro Poroshenko and the leaders of Russia, Germany and France having been postponed.
Heavy artillery and shelling shook the area around the airport in the rebel bastion of Donetsk, while pro-Kremlin insurgents had earlier been accused of unleashing a massive rocket assault.
Oleksandr Turchynov, secretary of Ukraine`s national security council, evoked a dire scenario before parliament, saying a resumption of major attacks by rebels could lead to a "large-scale continental war" if Russian troops were to also become more directly involved.
But he also spoke of another possibility -- one described by a number of analysts as well -- in which Russia would continue to prevent eastern Ukraine from being stabilised, causing the ex-Soviet republic to exhaust its economic and defence resources.
Such a strategy would aim, according to analysts, at keeping Ukraine from integrating more closely with the West, with the country already suffering economically and requiring billions of dollars from world lenders.
Russia strongly denies sending troops and weapons into the war zone despite witness claims to the contrary. On Thursday, Turchynov claimed some 8,500 Russian army soldiers were backing around 30,000 rebels in eastern Ukraine.
He also alleged that some 52,000 Russian troops were stationed near the Ukraine border along with tanks, armoured vehicles, artillery systems and combat helicopters.
In the face of the alleged threat, Ukraine on Thursday adopted a law allowing for three waves of reservist troop mobilisation this year, with the first, involving some 50,000 people, set for January 20.
The moves drew a sharp response from the separatist leader in Donetsk.
"Ukraine is preparing for war," Alexander Zakharchenko told reporters. "We are ready to respond adequately. We are not weak."The fresh violence comes after weeks of relative calm and served as a reminder of the war`s worst clashes last summer. In the most recent violence, two Ukrainian soldiers and two civilians were killed, Ukrainian authorities said.
Intense fighting shook the airport in the rebel bastion of Donetsk, an area repeatedly wracked by clashes. Observers from the Organisation for Security and Co-operation in Europe were due to visit the area later Thursday.
Witnesses reported hearing heavy artillery and bombings near the airport, while about 15 rebels and a few Ukrainian soldiers could be seen in the street trying to negotiate a pause in the violence themselves.
The bus tragedy on Tuesday at a checkpoint at the entrance to the city of Volnovakha shook the nation, with an online "Je Suis Volnovakha" campaign spreading, an imitation of the rallying cry used in the wake of the attack on the Charlie Hebdo satirical newspaper in Paris. Poroshenko himself used it in a Facebook post.
On Thursday, a minute of silence was observed at noon and flags were flown at half-mast.
Poroshenko had earlier discussed the tragedy in telephone conversations with French President Francois Hollande and German Chancellor Angela Merkel.
Both sides have traded blame over the bus tragedy, while Poroshenko argued that ultimate responsibility rested with Russia due to its alleged support of the rebels.
The rocket strike and renewed violence damaged Poroshenko`s efforts to set up a peace summit where Putin could personally sign a truce under which the Kremlin assumes responsibility for disarming the militias and dispelling their independence claims.
A lack of progress in talks between ministers in Berlin this week led to the summit being postponed. On Thursday, Kazakh President Nursultan Nazarbayev and Poroshenko agreed that the delayed talks set for the Kazakhstan capital Astana will take place at the end of the month.