At least 12 pro-Russia separatists died Monday in clashes at a Ukrainian border guards base and at the militant-occupied Luhansk regional government headquarters in another surge in fighting before Saturday's presidential inauguration.
Five militants were killed in daylong fighting at the guards base in Mirny, a Luhansk suburb, when as many as 500 gunman attacked it with rocket-propelled grenades, some firing from rooftops of apartment buildings to deter return fire, Ukrainian military officials said.
The interim government in Kiev, the capital, deployed fighter jets to drive the gunmen from the base, provoking claims by Russia's state-run media that the aircraft strafed the separatists' stronghold a few miles away in central Luhansk.
Russia Today television and the RIA Novosti news agency said an air-launched explosive penetrated a fourth-floor window in the five-story regional government building, which the separatists seized two months ago. The Russian reports said five people were killed in what was described as an airstrike. A Luhansk regional health service report later said seven died in the explosion and fire.
A Ukrainian military spokesman, Vladyslav Selezniov, denied that the government had fired from the air and speculated that the militants had unleashed an antiaircraft round on the building in a failed attempt to shoot down the fighter jets providing cover for troops at the nearby border guards base.
Separatists in Luhansk and the neighboring Donetsk region held makeshift referendums among their supporters on May 11 and proclaimed the territory they hold independent of Ukraine.
U.S. and European Union officials accuse Russian President Vladimir Putin of instigating the insurgency against Kiev, and separatist casualties in major attacks carried out over the last two weeks have shown that the majority of those killed were citizens of Russia or Russian-occupied areas of neighboring Georgia.
The militants have stepped up their attacks in the week since Ukrainians elected billionaire chocolate maker Petro Poroshenko as president. He is to be inaugurated Saturday, ending three months of provisional leadership that the Kremlin has refused to recognize since the Feb. 21 ouster of President Viktor Yanukovich.
Though armed clashes have intensified, the economic war between Moscow and Kiev was somewhat defused Monday when Russia's Gazprom monopoly agreed to give Ukraine another week to pay for its June gas supplies. At European Union-mediated talks in Brussels, Gazprom Chief Executive Alexei Miller acknowledged a $786-million payment from Kiev owed for natural gas deliveries this year.
Gazprom contends that Kiev still owes $3.5 billion for previous deliveries. That figure is based on an 80% rate hike that followed the ouster of Kremlin-allied Yanukovich and Moscow's annexation of Ukraine's Crimean peninsula, where a previous leasing agreement included a hefty discount for Ukrainian gas purchases.