Pro-Russian separatists unleashed a series of bomb attacks Tuesday in eastern Ukraine, leveling a key airport in Donetsk, killing 12 in an attack on a passenger bus and almost certainly dooming a short-lived cease-fire, according to reports.

A senior State Department official confirmed to Fox News that the separatists destroyed the government-held airport in eastern Ukraine Tuesday afternoon.

The facility has been “flattened” and the air control tower was “decimated,” the official said. “They are now fighting over rubble.”

Maria Ivanovna, a local retiree, told The Associated Press she has become desensitized to the blasts and drew an arc with her arm to show how shells fly over her home toward the airport.

"We will survive the same way we did after World War II. Ration cards for bread; 11 ounces for children; 800 grams for factory workers and 1,200 grams for miners," she said.

U.S. State Department spokeswoman Marie Harf said on Tuesday the bus attack was "egregious" and blamed Russia for helping to arm the separatists.

“We again call on Russia to fulfill its commitment under the Minsk Agreement, which includes ceasing its substantial military support to the separatists, restoring Ukrainian sovereignty over the international border between Ukraine and Russia, releasing all hostages, and working toward the peaceful resolution of the conflict in Eastern Ukraine,” Harf said.

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 tranquility 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 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 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 says 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 license plates heading for Donetsk, suggesting that new supplies were coming in for the rebels.

NATO's top commander, Gen. 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.

Ukraine 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.

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 witness 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.

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 been 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.

A 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 radicalizing the mood. 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.

Fox News' James Rosen and The Associated Press contributed to this report.