“The Russian side must be reminded about their obligations under the Geneva agreement to bring all necessary influence to bear on separatists to clear illegally held buildings, unblock roads, lay down arms and prevent any bloodshed,” the Ukrainian statement said.

Life News, a Russian tabloid television channel, reported that pro-Russian militants had found a business card of the leader of Right Sector, Dmytro Yarosh, among the belongings said to have been left behind by one of the attackers, along with guns and stacks of American dollars.

The Russian Foreign Ministry issued a statement saying that “as a result of an attack by insurgents of the so-called Right Sector, innocent civilians have died.” It added that pro-Russian fighters had found “aerial photographs of that district” and Right Sector emblems in cars seized from the attackers. “The Russian side is indignant at this provocation of the insurgents, which shows the Kiev authorities’ unwillingness to rein and disarm the nationalists and extremists,” it said.

The authorities in Kiev, Ukraine’s capital, ridiculed and rejected the Russian statement.

The checkpoint was a confused scene after the shootout. The remains of a pickup truck and a sport utility vehicle sat in the center of the road, incinerated except for two unburned out-of-town license plates: one screwed onto a fender, the other merely set on a fender.

Bullet holes in the pickup truck’s driver-side door showed that the truck had been fired on from the side or from behind as it faced the checkpoint. The men at the checkpoint said they had been fighting off an attack.

Journalists saw two bodies near the shooting, and officials in the town morgue said three people had died of gunshot wounds. The Ukrainian police said three people had been killed and four wounded. Pro-Russian militants said three of their members and two attackers had died.

Soon enough, each side interpreted the tableau of violence in its own way. The mayor of Slovyansk, Vyachislav Ponomaryov, who was appointed by the pro-Russian militants, imposed a curfew and asked President Vladimir V. Putin of Russia to send Russian peacekeepers.