Donetsk - An attack on a passenger bus in eastern Ukraine killed 11 people on Tuesday, possibly dealing the final blow to hopes that a short-lived and shaky cease-fire could take hold.Across Donetsk, the city that Russian-backed separatists call their capital, explosions and the sound of shells whistling overhead are again unnerving the local population. The holiday period was spent in relative tranquillity after a new truce was called in December between government troops and Russian-backed militia. But by late last week, that uneasy calm was steadily unravelling.In the single largest loss of life so far this year, civilians travelling on a commuter bus from Donetsk were killed on Tuesday afternoon by what Ukrainians say were rockets fired from a Grad launcher in rebel territory. Regional authorities loyal to Kiev said the bus was passing a Ukrainian army checkpoint at the time, putting it in the line of fire.Leading rebel representative Denis Pushilin denied responsibility for the attack.The warring sides are now trading accusations over who is responsible for the breakdown in the truce that led to Tuesday's deaths.Military spokesman Andriy Lysenko said that separatist attacks in recent days suggest an attempted onslaught to push back the frontline is under way. Separatist leader Alexander Zakharchenko said Ukraine's armed forces unilaterally resumed hostilities and that his fighters would respond in kind.An AP reporter over the weekend saw a convoy of around 30 military-style trucks without licence plates heading for Donetsk, suggesting that new supplies were coming in for the rebels.Nato's top commander, General Philip Breedlove, said Tuesday that there has been a continued resupply and training of rebel forces over the holiday period."Those continue to provide a concern and something that we have to be thinking about," Breedlove said.Rebels well equippedUkraine and the West have routinely accused Russia of being behind such consignments. Moscow flatly rejects the charges, although rebel forces are so well-equipped with powerful arms that the denials have become increasingly hollow."These are separatists that are clearly backed by Russia," US State Department spokesperson Marie Harf said Tuesday, adding that Russia must do more to stop the violence and restore Ukrainian sovereignty.In the rebel-held Donetsk suburb of Makiivka, the thrash of outgoing mortars shakes still-inhabited neighbourhoods on a daily basis. Separatists have consistently denied using residential areas for cover, but there are ample eyewitness accounts undermining those claims.Ukrainian responses to artillery lobbed out of Donetsk are woefully inaccurate and regularly hit houses and apartment blocks, often killing people inside. The separatist military headquarters in Donetsk said Tuesday that 12 people had been killed and another 30 injured in the preceding three-day period. It did not specify who had been killed.There is little sign of life in Makiivka these days. People rush home from work or aid distribution points and occasionally come out of shelters to exchange information about where shells are landing.Maria Ivanovna, a local retiree, told AP she is inured to the blasts and drew an arc with her arm to show how shells fly over her home toward the government-held airport on the northern edge of the city."We will survive the same way we did after World War II. Ration cards for bread. 300g for children, 800g for factory workers and 1 200g for miners," she said.A senior UN human rights official said this week that developments look poised to go in one of three directions — a frozen conflict, an escalation in violence or an evolution to sustainable peace."In case of frozen conflict, we will more or less continue to be seeing [the same] human rights violations that we have facing so far," the official said, speaking on condition of anonymity because of the sensitivity of the issue. "But in case of escalation of hostilities, which is quite possible, we could also be seeing further internationalization of the conflict and far more human rights violations and suffering."The grimmest of outcomes appears most likely.MobilizationA hoped-for round of peace negotiations this week between the leaders of Russia, Ukraine, Germany and France has been put on ice — possibly indefinitely.Ukrainian military authorities talk like they are bracing for the long-haul and Tuesday laid out plans for a new round of mobilization.Volodymyr Talalai, deputy head of the army's mobilization planning, said recruits will be drawn from all regions of the country. He gave no figure for how many people will be mobilized, but said that the primary aim of the upcoming drive is to enable the rotation of forces.Unremitting violence is radicalising the mood. One resident of Donetsk's Petrovsky neighbourhood - one of the most intensely bombed — said she took up arms and joined the separatist army after a rocket hit a home in her neighbourhood."A Grad landed ... and people were killed and blown to bits," she said, giving her name only as Vera. "How were we supposed to react? We are out here defending ourselves."Wearing a balaclava and cradling an automatic rifle, Vera said her 19-year old son too wanted to sign up, but that she refused to let him."I told them I would rather go myself than let my child do it," she said.