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 proved to be an agile foe.
Ukraine, meanwhile, an-nounced that President-elect Petro Poroshenko will be sworn in June 7, less than two weeks after his overwhelming victory in special balloting that it was hoped would ease tensions in the deeply divided country. Mr. 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, said 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.
Mr. 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 United States hasn't verified details of the incident, but he added that Washington is concerned because it indicates that 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 foes, fears that fighting could degenerate into brutal urban warfare have held back authorities from ordering an all-out assault.
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 beyond deepening civilian distrust toward the government 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 her family's safety. "I have four children. It is terrifying being here, because I am afraid for their lives."
Russia's Foreign Ministry denounced 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." The ministry said it would be impossible to restore peace without Kiev's halting the military operation against the rebels and withdrawing its troops. 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 separatists' appeal 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. Moscow denies that, saying it has no influence over the rebels, who insist they are only protecting eastern Russian-speakers interests.
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 various 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 defense analyst Mykola Sungurovskiy at Kiev's Razumkov Center.