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SLOVYANSK, Ukraine -- In another devastating blow to Ukraine's armed forces, rebels shot down a troop helicopter Thursday, killing at least 12 soldiers, including a general who had served in the Soviet army and was in charge of combat training.
The loss underscored the challenge Ukrainian forces face in fighting a guerrilla-style insurgency that has proven to be an agile foe.
Ukraine, meanwhile, announced that President-elect Petro Poroshenko will be sworn in June 7, less than two weeks after his overwhelming victory in special balloting that was hoped would ease tensions in the deeply divided country. Poroshenko has promised to negotiate with representatives in rebellious eastern Ukraine but also has vowed to uproot the pro-Moscow rebels who want the region to join Russia.
The Mi-8 helicopter was downed on the outskirts of Slovyansk by rebels using a portable air defense missile, according to Oleksandr Turchynov, Ukraine's acting president, in remarks to parliament in Kiev. Slovyansk, a city of 120,000 people, has become a focal point for the insurgency and has for weeks been encircled by Ukrainian troops.
Turchynov said the helicopter was rotating troops into a checkpoint when it came under rebel fire. Among the dead was Gen. Serhiy Kulchytskiy, who the Interfax news agency said had once served in the Soviet army and was in charge of training Ukraine's National Guard.
White House spokesman Jay Carney said the U.S. hasn't verified what happened in the incident, but he added that Washington is concerned because it indicates the separatists still have access to advanced weapons and are getting help from outside Ukraine, alluding to Russia.
While Ukrainian forces may be better equipped that their opponents, fears the fighting could degenerate into brutal urban warfare have so far held authorities back from ordering an all-out assault.
"It is extremely difficult to fight against guerrillas. You just cannot destroy them. They are not regular troops," said Igor Sutyagin, a research fellow at the London-based Royal United Services Institute. "It's the classic problem which Russia had in Chechnya and the United States had in Vietnam."
The Ukrainian government has been waging a military campaign in the Donetsk and Luhansk regions to try to put down the uprising by gunmen who have taken over public buildings and set up checkpoints. Dozens have been killed on both sides, including on Monday, when Ukrainian forces used fighter jets and helicopter gunships to dislodge rebels from the airport outside the city of Donetsk, the regional capital.
In recent days, Ukrainian troops have been using mortars to try to retake Slovyansk, causing civilian casualties and prompting some residents to flee. The tactic has produced few immediate results other than deepening distrust toward the government in the city and instilling general fear.
"They are shooting at us from grenade launchers. We hear explosions. The windows of our house are shaking," said Olga Mikhailova, who said she was leaving Slovyansk for the safety of her family. "I have four children. It is terrifying being here, because I am afraid for their lives."
Russia's Foreign Ministry denounced the use of aircraft and artillery against the rebels and demanded that Kiev end a "fratricidal war and launch a real political dialogue with all political forces and representatives of the regions."
It would be impossible to restore peace without Kiev halting the military operation against the rebels and withdrawing its troops, the ministry said. It urged the West to use its influence with Kiev to "stop Ukraine from sliding into a national catastrophe."
In an apparent bid to de-escalate tensions and avoid a new round of Western sanctions imposed after Moscow annexed the Crimean Peninsula in southern Ukraine, Russian President Vladimir Putin has ignored the appeal by the separatists to join with Russia. His spokesman, Dmitry Peskov, said Moscow has agreed to send "humanitarian aid" to eastern Ukraine.
Kiev condemns the insurgents as "terrorists" bent on destroying Ukraine and accuses Russia of fomenting the unrest. Russia denies that, saying it has no influence over the rebels, who insist they are only protecting the interests of Russian-speakers in the east.
The Ukrainian offensive has been hindered by a lack of experience and poor communication among its troops -- a mixture of soldiers, police, a newly formed National Guard and a number of often unaccountable volunteer battalions.
"As they have gained experience, they are becoming more efficient. But this has been limited by lack of cooperation, organization and coordination between divisions," said Mykola Sungurovskiy, a defense analyst with the Kiev-based Razumkov Center.
Poor coordination was on display May 23 when an insurgent attack on a government checkpoint in the town of Volnovakha resulted in an airstrike that killed 16 soldiers in an apparent case of friendly fire.
Disorganization plagues the rebels, too. Dozens of fighters from the insurgents' Vostok Battalion briefly surrounded the separatists' headquarters in Donetsk on Thursday in the most serious instance of infighting seen among the militants.
The standoff apparently was provoked by anger in the battalion, which is understood to be heavily made up of men from Russia's North Caucasus, at reports of their allies looting a supermarket near the Donetsk airport after Monday's deadly battle. Several dozen Vostok Battalion militiamen, including 34 Russian citizens, were killed in the fight for the airport.
The confrontation ended with the militiamen seizing the looted goods and bulldozing away the barricades that have stood outside the administration building since early April, when the self-styled Donetsk People's Republic announced its formation.
The morale among Ukrainian armed forces could also become an issue. In one episode reported this week by NTV, a pro-Kremlin Russian broadcaster, the parents of conscripts descended upon a Ukrainian Interior Ministry base in the Luhansk region to take their sons home.
An announcement Thursday by acting Defense Minister Mykhailo Koval that no troops involved in eastern Ukraine would be rotated out of the region could likely sour moods further. Koval blamed a lack of personnel.
Political analyst Vladimir Fesenko argues that Ukraine's military leaders might have wanted to push for maximum results from the offensive in the Donbass, as the eastern region is called, before Poroshenko takes office.
"The Ukrainian generals wanted to show Poroshenko they could act more effectively," said Fesenko, who is based in Kiev. "If operations in Donbass are ineffective, then Poroshenko will come to power and appoint new people. This is why the Ukrainian military leadership wanted to show Poroshenko that they could work effectively."
Leonard reported from Donetsk, Ukraine. Laura Mills in Kiev, Ukraine, Vladimir Isachenkov in Moscow also contributed to this report.