As the Ukrainian army moves to tighten its grip around the major separatist stronghold of Donetsk, the government may soon face a serious dilemma: if the army surrounds the city and the rebels choose not to flee, it could lead to the kind of urban warfare Kiev isn't militarily equipped for and civilian casualties that could prove politically devastating.

So far, most clashes between the government and the pro-Russian separatists have occurred outside urban areas in eastern Ukraine. But with Kiev making steady advances on the narrow corridor connecting Donetsk to the rest of rebel-held territory, observers fear that the fighting could descend into bloody street battles in a city where hundreds of thousands of residents remain and separatist leaders speak of a fight to the death.

"We are getting ready for an active defense of the city, our equipment is spread out throughout Donetsk, often hidden or disguised," says Roman Lyagin, a minister in the separatist's self-proclaimed government. "To destroy our tanks, rocket launchers, the Ukrainians will have to destroy the city."

It is a scenario that Kiev has publicly said it wants to avoid, which may mean they intend a protracted siege of the city designed to deplete the insurgents' supplies, cut them off from allies in Russia, and prompt some of them to lay down their arms. Any urban combat would be difficult to pursue for Ukrainian forces as the insurgents have had months to reconnoiter this city and fortify their positions.

"The Ukrainian armed forces are not trained and are not equipped to do this kind of job and a long, drawn-out battle in Donetsk could play against them," said Oleksiy Melnyk, a military analyst and former official in Ukraine's ministry of defense. "Every day the fighting goes on, there are more and more casualties and more and more infrastructure is destroyed. The government knows it needs to end this quickly."