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.

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.

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."