NOVOBAKHMUTIVKA, Ukraine — Three bridges on key roads leading into Donetsk were blown up Monday — an apparent attempt to slow down any possible assault by Ukrainian forces on the country's main rebel-held city.

Fighters driven out of Slovyansk and other eastern towns by the Ukrainian army had regrouped in Donetsk, a city of 1 million where pro-Russia separatists have declared independence under the Donetsk People's Republic. Pavel Gubarev, the region's self-described governor, had promised "real partisan war around the whole perimeter of Donetsk" before thousands of supporters at a rally Sunday.

It was not exactly clear who blew up the highway and train bridges Monday, but their destruction would most benefit the rebels.

In the village of Novobakhmutivka, where a rail line crosses over a highway out of Donetsk, an 11-wagon cargo train was perched perilously on the collapsed bridge. The road leads toward Slovyansk, a former insurgent stronghold that was recaptured Saturday by Ukrainian troops after intense fighting.

Anatoly Krasov, who was driving along the road Monday, said he saw an explosion before the bridge collapsed with a large cargo train on it. He said a group of men dressed in the camouflage uniforms often worn by the rebels then got into their cars and drove back toward Donetsk.

Two other bridges on roads leading from Slovyansk to Donetsk were also destroyed Monday in the villages of Zakitne and Seleznevka, the Road Transportation Agency of Donetsk Region said.

The insurgents control the regional administration building in Donetsk and checkpoints on the city's outskirts. They also face little internal resistance from police forces or government officials in the city, who have done nothing in recent months to hinder their free movement around Donetsk.

But it is unclear whether they will be able to put up major resistance in the face of a Ukrainian attack. Ukrainian forces demonstrated their superior firepower in repelling a rebel attempt to take control of Donetsk Airport in late May, a battle that left dozens of rebel fighters dead. Many residents have fled the city and the streets are often deserted but for the rebels.

Still, experts say that capturing Donetsk would be much more difficult than retaking Slovyansk, a city ten times smaller, and it could require the type of street-to-street urban warfare that would favor the rebels, not government troops.

There was no word Monday from Ukrainian President Petro Poroshenko, who had promised to start negotiations on a new cease-fire last week.

Battles between Ukrainian forces and pro-Russian separatists have left over 400 people dead and thousands homeless since they began in early April.

Ukraine and the West have accused Russia of fomenting the insurgency by sending troops and weapons, including tanks and rocket launchers, something Moscow denies. Putin has so far resisted demands at home and by the rebels to come to their aid, wary of having more Western sanctions slapped on Russia.

On Monday, Russia's Foreign Ministry made its first statement about Slovyansk since the city fell. It tiptoed around the rebels' defeat, only mentioning Slovyansk as part of a long list of civilian casualties.

Russia urged the European Union to put new pressure on Ukraine, which it accused of waging a "massive military operation which has resulted in the deaths of peaceful people."

Laura Mills in Moscow contributed reporting.