Fighting broke out again late Thursday between Ukrainian troops and pro-Russia rebels around the international airport in Donetsk, Russian and Ukrainian media reported.
Since losing several strongholds to resurgent government forces last week, separatist rebels have renewed their attacks on Ukraine's second-largest airport, a key transport hub in eastern Ukraine that has been closed for two months.
A Ukrainian security spokesman said in Kiev, the capital, that a Wednesday night mortar barrage by rebels attempting to take control of Sergei Prokofiev International Airport was defeated by the Ukrainian troops who have occupied the facility since mid-May.
But fresh bursts of gunfire and artillery could be heard Thursday night, the Russian news agency Itar-Tass reported.
"Self-defense forces are attacking the Ukrainian army positions at the airport. In response the Ukrainian side has continued firing mortars into the adjacent territory," Dmitry Gau, a military advisor to the separatists, was quoted as telling Russia's Interfax news agency.
Ukrainian government troops last weekend recovered control of Slovyansk, Kramatorsk and a few surrounding villages in Donetsk and Luhansk regions.
"Today, the Ukrainian military controls 13 out of the 18 districts of the Donetsk region, and 10 out of the 18 districts of the Lugansk region," National Security and Defense Council chief Andriy Parubiy told government leaders in Kiev on Thursday.
News agencies with reporters near the battlegrounds say the shift in fortunes in the conflict has sparked infighting among the separatist groups scurrying to recruit reinforcements and appealing to Moscow for aid in fighting off the government.
Igor Strelkov, the Russian special forces officer who has been leading the military campaign against rule from Kiev, withdrew the fighters from Slovyansk and Kramatorsk when they were about to be overrun by Ukrainian troops. The rebels are now regrouping in Donetsk, a city of a million residents that is the heart of Ukraine's Donbass industrial basin, while allied militias continue to attack government troops in the more rural regions.
Separatist leader Alexander Borodai disputed the reports of infighting in comments reported by the Associated Press. But his warning to Russia that another wave of civilian refugees was about to flood across the border appeared to be a veiled call for Kremlin reinforcements to rescue the anti-Kiev rebellion.
"This will be hundreds of thousands of people. This is an inevitability we have to face, to avoid civilian casualties," Borodai told reporters, according to a BBC report from Donetsk.
Another leader of the so-called People's Republic of Donetsk, Pavel Gubarev, lamented a day earlier that Russia, which Kiev authorities accuse of fomenting and arming the rebellion, has failed to respond to the separatists' calls for reinforcements to fight off the resurgent government forces.
Meanwhile, Russian authorities rejected Ukrainian accusations that they had abducted a female Ukrainian pilot captured by the separatists near Luhansk last month and taken her across the border to Russia.
Nadezhda Savchenko, a veteran of Ukraine's peacekeeping force in Iraq, was charged Wednesday with complicity in the deaths of two Russian journalists killed by a mortar attack on a rebel checkpoint near Luhansk. The Russian Investigative Service, an FBI-like agency, said she was captured after entering Russian territory without documents and disguised as a refugee. A Ukrainian military spokesman denounced that scenario as "absurd."
"Her case will be decided under the law, taking into account the fact that she ended up on Russian territory and we have the perfect right to prosecute foreign citizens in our own country,” Russian Foreign Ministry spokesman Alexander Lukashevich told reporters at a briefing in Moscow.
Savchenko is being detained in the southern Russian provincial city of Voronezh pending trial on charges that she tipped off Ukrainian forces to the journalists' whereabouts just ahead of the June 17 artillery barrage. Itar-Tass said Thursday that a local court had ordered she remain jailed at least until Aug. 30.
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