By Natalia Zinets and Alessandra Prentice
KIEV (Reuters) - Ukrainian President Petro Poroshenko said on Thursday his troops were holding the line against separatists and Russian forces after a sharp increase in attacks and the withdrawal of government defenders from Donetsk airport.
Poroshenko, who told the World Economic Forum on Wednesday that Russia had 9,000 troops inside Ukraine, met defense chiefs to work on a plan to "regroup and stop aggression."
"Across all front lines we are firmly holding our positions," he told the meeting, at which he reported that enemy attacks were 10 times more intensive than before.
Earlier on Thursday at least eight civilians were killed when a trolleybus was hit by an artillery shell or mortar at a public transport stop in the southern district of Donetsk, which is largely controlled by separatists.
Poroshenko's comments echoed a report from NATO that said fighting in separatist territories was now more intense in some places than it was before September's Minsk ceasefire agreement.
The missile strike that destroyed the trolleybus followed a night of intense fighting at the city's main airport.
Kiev said 10 Ukrainian soldiers were killed overnight, six at the airport complex, a symbolic target where a small group of government defenders had been holding out against Russian-backed separatists for months.
A military spokesman said government forces had withdrawn from the airport's new terminal, the core of the complex, in what appeared to be a setback for Ukrainian forces and for Poroshenko himself, who had said the airport should not be surrendered.
Rebel leader Alexander Zakharchenko urged local residents to gather at the scene of the bus attack, promising them the opportunity to confront captured Ukrainian servicemen. Separately, video footage showed a handcuffed prisoner being abused and punched by onlookers near the wrecked bus.
Regional officials said eight civilians had died. Rebels put the number higher.
Ukrainian Prime Minister Arseny Yatseniuk blamed separatists, who Kiev says are being armed by Moscow. Russia, which denies any direct involvement, described the attack as a "crude provocation" by Kiev to undermine peace efforts.
The United Nations condemned the attack and said it did not know who was responsible.
"That trolleybus attack clearly appears to us as being a targeted attack," United Nations spokesman Stephane Dujarric told reporters in New York. "Targeted attacks against civilians constitute a grievous violation of international humanitarian law and must be investigated."
A spokeswoman for the U.S. State Department said the incident was being investigated by the Organization for Security and Co-operation in Europe, but the United States condemned violence that had cost innocent lives.
Jen Psaki said it showed the need to implement an agreement on establishing security zones between pro-Russian fighters and Kiev's forces reached in talks involving Russia, Ukraine, France and Germany in Berlin on Wednesday.
Psaki said Ukraine had the right to defend its own territory and that Russia and Russian-backed separatists were responsible for the preponderance of violations.
"But we certainly expect both sides to abide by the agreement," she told a regular news briefing.
(Additional reporting by Pavel Polityuk in Kiev, Michelle Nichols in New York and David Brunnstrom in Washington; Writing by Richard Balmforth; Editing by Mark Trevelyan and Lisa Shumaker)