The missile strike, which wrecked a trolleybus and blew out windows nearby, followed a night of intense fighting at the city's main airport and diplomatic talks in Berlin involving Ukraine and Russia.
President Petro Poroshenko, returning early from Switzerland where he told the World Economic Forum that Russia now had 9,000 troops inside Ukraine, said he and defence chiefs would work on a plan to "regroup and stop aggression".
"We'll decide how to stop the operation of terrorist forces and Russian regular troopS," he said in an online statement.
Rebel leader Alexander Zakharchenko urged local residents to gather at the scene of the trolleybus attack, promising them the opportunity to confront captured Ukrainian servicemen. Separately, video footage showed a handcuffed Kiev prisoner being shouted at and punched by civilians near the trolleybus.
Military spokesman Vladislav Seleznyov said 10 Ukrainian soldiers were killed overnight, six at the airport complex, a symbolic target where a small group of government defenders have been holding out against Russian-backed separatists for months.
The spokesman later said government forces had withdrawn from the airport's new terminal, the core of the complex.
"We left the new terminal because it looks like a sieve and there's simply nowhere to hide there," Seleznyov said.
It was not clear what sort of missile hit the trolleybus or who fired it. Each side blamed the other.
A Reuters cameraman saw six bodies near, and inside, a gutted trolleybus in Bosse, a southern district of the city. A video clip showed the hulk of the trolleybus with its windows blown out and smoke pouring from a nearby passenger car.
Regional officials said eight 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 at undermining peace efforts.
Modest progress appeared to have been made in four-way talks late on Wednesday in Berlin involving Ukraine, Russia, Germany and France on the crisis.
German Foreign Minister Frank-Walter Steinmeier said foreign ministers had agreed a "demarcation line" between pro-Russian fighters and Kiev's forces from which withdrawal of heavy weapons could start. There were no details.
