An attack on a passenger bus in eastern Ukraine killed 11 people Tuesday, dealing what many believe to be the final blow to a short-lived and shaky cease-fire that never fully took hold.

Across Donetsk, the city that separatists call their capital, explosions and the sound of shells whistling overhead unnerved the local population. The holiday period was spent in relative tranquility after a new truce was called in December between government troops and Russian-backed militia. But by late last week, the uneasy calm was steadily unraveling.

In the single largest loss of life so far this year, civilians traveling on a commuter bus from Donetsk were killed Tuesday afternoon by what Kiev said were rockets fired from a Grad launcher in rebel territory. At least a dozen more people were injured.

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, as various rebel sources accused the Ukrainian military of firing on its own checkpoint. But analysis of the shell’s trajectory as well as rebel social media chatter – which initially took credit for the attack before it became clear civilians had been killed – lend credence to Kiev’s theory, according to The Interpreter blog, which monitors the conflict in Ukraine.

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.

Reporters over the weekend observed a convoy of military-style trucks without license plates heading for Donetsk, suggesting that new supplies were coming in for the rebels. NATO's top commander, Gen. Philip Breedlove, added Tuesday that there was a continued resupply and training of rebel forces over the holiday period.

Ukraine and the West have routinely accused Russia of being behind such consignments. Moscow flatly rejects the charges, but rebel forces are so well-equipped with powerful arms that the denials have become increasingly hollow.

In the rebel-held Donetsk suburb of Makiivka, the thrash of outgoing mortars shakes still-inhabited neighborhoods 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.

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. 300 grams (11 ounces) for children, 800 grams for factory workers and 1,200 grams for miners," she said.

A senior U.N. 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."

Hopes for a negotiated solution faded this week when a proposed round of peace negotiations between the leaders of Russia, Ukraine, Germany and France was put on ice — possibly indefinitely.

Ukrainian military authorities talk like they are bracing for the long haul and on Tuesday laid out plans for a new round of mobilization. Volodymyr Talalai, deputy head of the army's mobilization planning, said recruits would be drawn from all regions of the country. He gave no figure for how many people would be mobilized, but said that the primary aim of the upcoming drive is to enable the rotation of forces.

In a televised address, Ukrainian President Petro Poroshenko announced new restrictions on movement in Ukraine’s east and said he would sign off on the new mobilization plans. Comparing Tuesday’s attack on the Volnovakha checkpoint with the Paris terror attacks of last week, Poroshenko later tweeted: “Mass killings in Paris and Volnovakha have very much in common. Civilized world must stand united against terror.”

Meanwhile, unremitting violence is radicalizing the mood on the ground. One resident of Donetsk's Petrovsky neighborhood – one of the most intensely bombed – said she took up arms and joined the separatist army after a rocket hit a home in her neighborhood.

"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.

Al Jazeera and wire services