Driven from their key stronghold in eastern Ukraine, pro-Russian separatists regrouped Sunday in the city of Donetsk, vowing in front of thousands of cheering supporters to renew their fight against the government in Kiev.

Ukrainian officials, meanwhile, vowed to regain more territory from rebels after seizing Slovyansk on Saturday. The city of about 100,000 had been the site of fierce fighting, and the government’s success there suggested that it might finally be making gains in the months-long battle against the insurgency.

On Sunday, Ukrainian forces shelled parts of the rebel-held town of Luhansk near the Russian border, hitting a battery-making factory and other buildings, Russia’s Itar-Tass news agency reported, quoting rebels in the town.

Regarding casualties, the news agency said only that some people were wounded. “People are hurriedly fleeing to bomb-shelters or are leaving the area that is being shelled,” it said.

“My order is now in effect — tighten the ring around the terrorists,” Ukrainian President ­Petro Poroshenko tweeted Sunday. “Continue the operation to liberate Donetsk and Luhansk regions.”

It is not clear whether the rebels have been permanently hobbled or whether the setback will bring help from Russia. Rebel fighters moved freely Sunday around central Donetsk, a city of 1 million and the capital of a region where the separatists have declared independence in the name of the Donetsk People’s Republic.

The insurgents control the regional administration building and checkpoints on the city outskirts, but Ukrainian forces demonstrated their superior firepower in repelling a rebel attempt to take control of Donetsk airport in late May. That battle left dozens of rebel fighters dead.

Igor Girkin, the defense minister of the separatist republic, told the Russian television channel LifeNews on Sunday that he would keep up the fight from Donetsk.

“We will continue the combat operations and will try not to make the same mistakes that we made in the past,” said Girkin, a Russian also known by his nom de guerre, Igor Strelkov. Ukrainian authorities have identified him as a veteran of the Russian military intelligence agency.

At Sunday’s rally, people waved flags from Russia and the Donetsk People’s Republic.

“We will begin a real partisan war around the whole perimeter of Donetsk,” Pavel Gubarev, the self-proclaimed governor of the republic, told the crowd. “We will drown these wretches in blood.”

But, he added, the insurgents could easily die in Donetsk if Russia does not come to their aid.

Rebel leaders have pleaded with the Kremlin for military assistance, and some prominent Russian nationalists have taunted President Vladimir Putin for not sending troops to the Russian-speaking area. Putin has resisted, threatened by additional Western sanctions.

Ukraine and the West have accused Russia of fomenting the insurgency by sending troops and weapons, including tanks and rocket launchers. Moscow has denied providing soldiers or military equipment, saying that Russians fighting in the east are private citizens.

Despite the bravado in ­Donetsk, the mood was dire at a rebel checkpoint on the city’s outskirts.

“We will fight to the end because we have nowhere left to retreat,” said one of the masked fighters, a 32-year-old former coal miner who would give only his first name, Artyom, because he feared retaliation. “I don’t want to fall into the hands of the Ukrainian authorities, those fascists.”

Artyom said that although the insurgents still hope for help from Russia, “the hope grows weaker with every day.”