SLOVYANSK, Ukraine — Ukrainian security forces killed three pro-Russian activists, wounded 13 and took 63 captive in a firefight overnight in the eastern city of Mariupol, the interim Ukrainian interior minister said on Thursday. The clash was the most lethal so far in the east of the country.

The minister, Arsen Avakov, said the protesters had tried to storm a base used by troops in the newly formed National Guard, which has drawn from volunteers who took part in last winter’s protest movement against the government.

A crowd made up of civilians as well as militants formed outside the base, he said. Mr. Avakov’s description of the conflict indicated another challenge for the new government, as in other cases where pro-Russian groups have seized administrative buildings and police stations.

In Moscow, the Russian Foreign Ministry threatened retaliation after the Kiev government said it would restrict entry of Russian men into the country. Kiev has accused the Kremlin of waging a shadow war in eastern Ukraine, infiltrating not only Russian troops without insignia but also Russian citizens to pose as pro-Russian activists.