Residential areas in the eastern Ukrainian town of Horlivka and a checkpoint at Ukraine’s Krasna Talivka border crossing were destroyed in two separate attacks. Ukraine’s defense council blamed separatists for the attacks as fighting continues in eastern Ukraine. (Divya Jeswani Verma/The Washington Post)

Residential areas in the eastern Ukrainian town of Horlivka and a checkpoint at Ukraine’s Krasna Talivka border crossing were destroyed in two separate attacks. Ukraine’s defense council blamed separatists for the attacks as fighting continues in eastern Ukraine. (Divya Jeswani Verma/The Washington Post)

Government troops were engaged in a pitched battle with rebels on Saturday just outside the separatist bastion of Donetsk and plan to advance next into the city that has been at the heart of the pro-Russian insurgency.

If the army succeeds in retaking Horlivka, a city of almost 300,000 people where fighting was fierce Saturday, they will be within a few miles of Donetsk. Rebels have held sway there since the spring, ruling what they call the Donetsk People’s Republic. Cars created roadblocks out of town Saturday, and the railway station was packed with people desperate to board the next train out.

The military already has ousted rebels from 10 surrounding villages and towns over the past week and blocked roads into and out of Donetsk to prevent supplies from entering the city, according to Andriy Lysenko, a spokesman for the Ukrainian Security and Defense Council.

“The next one will be Donetsk,” Lysenko said, making a bold prediction: “The city will be liberated.”

Ukrainian officials have sounded increasingly confident in recent days, even though 15,000 Russian troops are believed to be massed at the border and gaining in men and materiel with every passing day. American and Ukrainian officials have said Moscow appears to be stepping up its support of the rebels, which include many Russian citizens, since a Malaysia Airlines passenger plane was downed on July 17 by a missile fired from separatist-held territory.

1 of 35 Full Screen Autoplay Close July 28, 2014 July 27, 2014 July 26, 2014 July 25, 2014 July 21, 2014 Skip Ad × Clashes break out in eastern Ukraine city View Photos Fighting between pro-Russian separatists and government forces in the rebel stronghold of Donetsk leaves five dead. Caption Kiev government officials are optimistic after Ukraine’s military ousts pro-Russian forces near the separatist bastion. A person looks at damage in Donetsk. Bulent Kilic/AFP/Getty Images Buy Photo Wait 1 second to continue.

Explosions from the battlefield can be heard at the unsecured crash site, where investigators say they are still finding human remains. A plane carrying the last of 227 coffins filled with body bags of passenger and crew from the crash left Kharkiv for the Netherlands on Saturday, though it’s not known how many remains are in each bag. That potentially leaves dozens of people unaccounted for.

Rebels said that they have given Dutch officials luggage and other personal effects belonging to crash victims, according to a statement cited by Russian news service Interfax.

Ukrainian President Petro Poroshenko said his country was not fighting a civil war in the east but “foreign mercenaries.”

“This is a real fight for the sovereignty of Ukraine, the territorial integrity of Ukraine, for the independence of Ukraine,” he said while awarding service medals to the National Guard. “It is not an internal conflict; it is Ukraine defending its territory from foreign mercenaries, from bandits and from terrorists.”

Russia, in turn, stepped up its rhetoric against the United States and Europe, accusing the United States of spreading lies and warning that sanctions imposed by the European Union are “endangering international security cooperation.”

A Foreign Ministry statement said the sanctions show Europe has “embarked on a complete turning away from cooperation with Russia on international and regional security, including the fight against the proliferation of weapons of mass destruction, terrorism, organized crime and other new challenges and threats.”

“We are convinced that such decisions will be enthusiastically received by international terrorists,” the statement added.

In a separate statement, the ministry accused the U.S. government of conducting “an unrelenting campaign of slander against Russia, ever more relying on open lies.”

The escalating tensions, within Ukraine and with its neighbor, have left many Ukrainians on edge.

The mayor of Kremenchuk, a town in central Ukraine, was fatally shot Saturday. The house of another mayor, in Lviv in western Ukraine, was damaged by fire from an antitank grenade launcher. That led many people to speculate on social media that the attacks were connected to the insurgency, though there was no evidence immediately available to prove that.

Karoun Demirjian reported from St. Petersburg. Alex Ryabchyn in Kiev and Karen DeYoung in Washington contributed to this report.