Ukrainian forces and separatist rebels have fought fierce gunbattles around Luhansk, with hundreds of rebels attacking a border guard base and Ukraine responding with air raids on rebel positions.
Pro-Russian separatists claimed on Monday that at least five people were killed and eight were wounded when Ukrainian jets dropped bombs on a government administration building they occupied in the city.
The explosion came after 500 separatists attacked and lay siege to a border guard base near Luhansk manned by 70 Ukrainian soldiers.
Al Jazeera's David Chater, reporting from the base, said those soldiers had been trying to control a flow of separatist volunteers crossing the border from Russia.
He said that the rebels appeared to be controlled from within the city, hence the air raid, but the action did not appear to be enough to save those in the base.
"Kiev is saying that they have ordered a full counter-terrorism operation, as they call it, using aviation. We have to wait and see if that it is going to be used against the fighters surrounding the border guard camp," said Chater.
At least five of the attackers were killed in fighting around the base, a spokesman for the guards told the AP news agency.
Rebels massed near the base had earlier promised border officers that they would be unharmed if they surrendered and laid down their arms, AP reported.
The initial attack was launched by about 100 rebels but the number of attackers swelled a few hours later.
Luhansk air raid
Rebel forces from the self-proclaimed Luhansk People's Republic claimed that Ukrainian jets had responded by dropping cluster bombs on the administrative buildings they occupied in the city.
A video showed an explosion, debris scattered around the building and men trying to move a body.
"Cluster bombs were used in today's air raid by the Ukrainian air force on the Luhansk region state administration," Russia's Itar-Tass news agency quoted the self-proclaimed Luhansk People's Republic.
Moscow accused Kiev of "crimes against its own people".
However Vladyslav Seleznyov, a spokesman for Ukraine's military, said the blast was caused when a separatist missle targeting a Ukrainian jet misfired.
"We destroyed two checkpoints in Luhansk. When the plane was turning back, they tried to shoot it down. A heat-seeking device was launched but it fell on the fighters themselves and destroyed part of the wall of the administration building," Seleznyov said.
Russia called an emergency meeting of the UN Security Council on Monday to introduce a resolution calling for an immediate ceasefire in Ukraine.
"This draft resolution will also contain a requirement to immediately create humanitarian corridors that will help civilians leave hostility zones, should they wish to do so," said Sergey Lavrov, Russia's foreign minister.
The move was immediately denounced by Ukraine and the United States.
"The very fact that the resolution is tabled by the Russian federation is cynical and immoral," said Ukrainian Ambassador to the United Nations Yuriy Sergeyev.
Sergeyev called on Russia to stop "mercenaries" from flooding into the eastern regions of the country.
Meanwhile, US President Barack Obama was set to land in Warsaw on Tuesday to open a European tour shaped by the Ukraine crisis, on which he was to reinforce US security guarantees to worried eastern European NATO members.