Pro-Russian fighters have converged on the major industrial city of Donetsk, a day after they were driven out of their key stronghold in eastern Ukraine.
Their commander said on Sunday that the fighters would regroup to renew their fight against the Ukrainian government.
Ukrainian troops on Saturday forced the rebels out of Slovyansk, a city of about 100,000 that had been at the centre of the fighting.
The success there suggests that the government may finally be making gains in the months-long battle against the rebels.
It was not yet clear, however, whether the capture of Slovyansk has permanently crippled the rebels.
Rebel fighters from Slovyansk and other towns taken over by the Ukrainian army were seen on Sunday milling around central Donetsk, the AFP news agency reported.
Donetsk is the provincial capital of a region with a population of about 1 million where the separatists have declared independence in the name of the Donetsk People's Republic.
Igor Girkin, the defence minister of the separatist republic, said in a video interview on Sunday with the Russian television channel Life News that he had arrived in Donetsk from Slovyansk.
"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 alias, Igor Strelkov. Ukrainian authorities identify him as a veteran of the Russian military intelligence agency.
Pro-Russian fighters also have been fighting Ukrainian troops in the neighbouring Luhansk region, which like Donetsk sits along the border with Russia.