Hundreds of pro-Russia rebels armed with automatic weapons and rocket-propelled grenades have mounted a day-long assault on a key government base used to co-ordinate the defence of the country's border with Russia, prompting the deployment of air support by government forces.
Border guards killed at least five rebels in repelling the attack on their base on Monday (local time), a spokesman for the border guard service said.
In the centre of Luhansk, some 10 kilometres away, a blast at an administrative building held by the insurgents claimed more lives. A health official for the Luhansk region told Interfax news agency that at least seven people had been confirmed dead in what rebels described as a government airstrike.
The government denied carrying out an airstrike and said the blast was caused by misdirected rebel fire from a portable surface-to-air missile launcher.
Russia's Foreign Ministry swiftly condemned what it said was a government attack on the rebel-held building and urged US Assistant Secretary of Defense Derek Chollet, who was visiting Kiev on Monday, to help calm unrest in Ukraine.
"We urge our Western partners to use their influence on Kiev, to stop Ukraine from descending into a national catastrophe," the ministry said in a statement.
Russia also called an emergency meeting of the UN Security Council to introduce a resolution calling for an immediate halt to the violence and talks to establish a cease-fire. Moscow has almost daily demanded that the Kiev government halt its military operations in the east, but it was the first time it has called for a Security Council resolution. It was unclear how much support the proposal would have.
The rebel assault on the government base continued into the night, ending about 9pm with the border guards succeeded in repelling the insurgents.
Earlier, rebels in camouflage had promised safe passage to the government troops if they surrendered and laid down their arms. The pro-Russian insurgents, who have seized government and police buildings across eastern Ukraine, have waged increasingly aggressive attacks on government-held checkpoints and garrisons in an attempt to seize weapons and ammunition from Ukrainian forces.
The attacks are deeply troubling for Ukraine's new government, whose president-elect, Petro Poroshenko, has pledged to crush the separatist movement in the east. The conflict has escalated markedly in the past week, with rebels attempting to seize a major airport and the shooting-down of a Ukrainian military helicopter.
Serhiy Astakhov, the spokesman for the border guard service, told The Associated Press that five rebels were killed and eight wounded in Monday's attack on the walled compound on the western fringes of Luhansk, a major city not far from the Russian border. He said seven servicemen were wounded, three seriously.
The initial attack by about 100 insurgents was met by gunfire from the border guards, and the number of attackers swelled to around 400 a few hours later, he said. Astakhov said the Ukrainian armed forces sent aircraft to the area, and at least one fighter jet was seen flying overhead.
An AP reporter saw about 40 rebel fighters, and one of them said that more than 200 were involved in the assault on the base.
At least one dead rebel fighter fell about a half-mile away from the base. Fellow insurgents approached and broke into tears as they viewed the body. One insurgent said the dead man was a leading rebel commander.
Heavy gunfire was heard in the area, and rebel fighters fired at least six rocket-propelled grenades at the government base from the rooftop of a residential building.
One insurgent fighter in uniform, who gave his name as Vlad Sevastopolsky, said pro-Russian militants surrounded the base and offered Ukrainian troops a safe corridor out, as long as they surrendered their weapons.
Vladislav Seleznyov, press secretary for Ukraine's operation against the rebels in the east, described the base as an important co-ordinating node for the border guards, and told The Associated Press that the attack may have been an attempt to disrupt communications.
He said there was another rebel attack on Monday on a government checkpoint in Slovyansk, a city in the Donetsk region that has been an epicentre of the pro-Russian movement. He said rebels set mines at a number of power plants in Slovyansk, and threatened to detonate them if the government moved on the city.
In the regional capital of Donetsk, meanwhile, gunmen from the self-proclaimed Donetsk People's Republic entered the office of the local newspaper and took away its editor, Leonid Lapa, his deputy, Valery Lapshin, told AP.
The gunmen told the journalists they were taking the editor in for questioning. He was released several hours later, Lapshin said.
For weeks, Russian-speaking eastern Ukraine has been the scene of deadly clashes between government troops and pro-Russian insurgents.
Many in Ukraine's east are suspicious of the new pro-Western government in Kiev, which came to power when President Viktor Yanukovych fled to Russia in February after months of street protests in Kiev. Protests in the east demanding greater freedom from the Ukrainian capital soon turned into a separatist movement, as the Luhansk and Donetsk regions declared independence following hastily called referendums.
In Moscow, the Russian Defense Ministry on Monday announced a military exercise involving the launch of high-precision missiles. The ministry said the maneuvers of the western military district will continue through Thursday and involve the deployment of Iskander surface-to-surface missiles.
Moscow didn't specify the areas where the exercise will be held and made no mention of the situation in Ukraine.