Ukraine's armed forces suffered devastating new losses Thursday, underlining the scale of the challenge the country faces in quelling a guerrilla-style insurgency that has proven to be agile and ruthless.

A rebel rocket attack brought down a military Mi-8 helicopter ferrying out troops, including a general, on the outskirts of Slovyansk, killing at least 12 people onboard.

Acting President Oleksandr Turchynov told parliament in Kyiv that rebels used a portable air defence missile against helicopter.

Slovyansk, a city of 120,000 people, has become a focal point for the armed pro-Russian insurgency and has for weeks been encircled by Ukrainian troops.

While Ukrainian forces may be better equipped than their opponents, fears that the standoff 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."

In recent days, Ukrainian troops have taken to deploying mortar shells in their bid 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 and instilling fear.

"They are shooting at us from grenade launchers, we hear explosions. The windows of our house are shaking," said resident Olga Mikhailova, who said she was leaving the city 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 on Thursday denounced the use of aircraft and artillery against the rebels and demanded that Kyiv 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 in Ukraine without ending the government's military action against the rebels and withdrawing Ukrainian troops from the east. It called on the West to use its clout with Kyiv to "stop Ukraine from sliding into a national catastrophe."

The Kyiv government condemns the insurgency as the work of "terrorists" bent on destroying the country and accuses Russia of fomenting it. Russia denies the accusations, saying it has no influence over rebels, who insist they are only protecting the interests of Russian-speakers in the east.

Ukraine's military effort has been hindered by a lack of experience in waging operations of the sort underway in eastern Ukraine.

The military, police troops, a newly formed National Guard and a number of often unaccountable volunteer battalions are all ostensibly operating under an "anti-terrorism operation," but it is clear communication has been poor. And lack of military prowess among the most freshly minted units often shows.

"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 Kyiv-based Razumkov Center. "There have been some cases where there was an attack but no reinforcement, or when 30 rebels were killed in one day and yet KAMAZ trucks are bursting across the Ukrainian border from Russia."

Bad coordination was most vividly on display when insurgents attacked a government checkpoint in the town of Volnovakha on May 23. Aerial reinforcements apparently shot on their own men. Sixteen soldiers died.

Rebels have also shown signs of disunity.

On Thursday afternoon, dozens of fighters from the insurgent Vostok Battalion surrounded separatist headquarters in the regional government building in Donetsk — the most serious insurgent infighting to date.

The standoff was apparently provoked by anger in the Vostok Battalion, understood to have many combatants from Russia's North Caucasus, at reports of looting by their allies from a supermarket near the airport, the scene of bloody clashes Monday. Several dozen Vostok Battalion militiamen were killed in that fighting.

The confrontation ended with the militiamen seizing the looted goods and bulldozing the barricades that have lined the building since early April, when the self-styled Donetsk People's Republic announced its formation.

On the government side, every deadly incident is only likely to sap morale among Ukrainian armed forces that have sometimes been shown to lack basic equipment.

In one episode this week, reported by pro-Kremlin Russian channel NTV, the parents of conscripts serving at an Interior Ministry base in Luhansk region, where rebels have also declared independence, descended en masse to take their sons home.