KIEV, May 6 – Ukraine on Tuesday continued to wage an offensive against pro-Russian separatists in eastern parts of the country, apparently seeking to disrupt a secession referendum planned in the region in four days.

Violence flared in the eastern port of Mariupil, with pro-Moscow militants telling Russian news agency Itar-Tass that that one person was killed and three wounded in an attack on a checkpoint.

Local website 0629.com.ua posted pictures of tires blazing outside the Mariupil city council building and thick smoke pouring over the town centre. Some streets were barricaded by buses.

Interior Minister Arsen Avakov said on Tuesday more than 30 separatists had been killed in fighting around Slavyansk, but there was no confirmation of such a figure from the pro-Russian rebels.

The next few days could prove decisive: separatists in the Donbas region say they will hold a referendum on secession on Sunday, similar to the one that preceded Russia's annexation of Crimea in March.

In Washington, U.S. Secretary of State John Kerry denounced any attempt at a vote as "bogus," saying: "We flatly reject this illegal effort to further divide Ukraine," Reuters reported.

“This is really the Crimea playbook all over again, and no civilized nation is going to recognize the results of such a bogus effort," he said in a news conference with EU foreign policy chief Catherine Ashton.

He again cautioned Moscow that Washington would impose more powerful sanctions designed to hurt Russia's economy if it tried to disrupt Ukraine's presidential election set for May 25.

Kerry suggested, however, that moving too soon on tougher sanctions before diplomatic efforts have been fully explored could backfire. He said he would meet ministers in Europe next week to discuss the next steps on Ukraine.

Two days before the separatists' referendum is Friday's annual Victory Day holiday celebrating the Soviet Union's triumph over Nazi Germany.

Moscow has been openly comparing the government in Kiev to the Nazis, and Ukrainian officials say they are worried that the day could provoke violence. In Moscow, there will be a massive parade of military hardware through Red Square, a Soviet-era tradition revived by Putin.

Since a pro-European government took power after the uprising that toppled pro-Russian President Viktor Yanukovych, Russian President Vladimir Putin overturned diplomatic convention by declaring Moscow's right to send troops across borders to protect Russian speakers.

Since Crimea's annexation, armed separatists have taken control of most of the Donbas, which accounts for about 15 percent of Ukraine's population and a third of industrial output.

Moscow has tens of thousands of troops massed on Ukraine's eastern frontier. The outbreak of violence in Odessa, hundreds of kilometers away near a Russian-occupied breakaway region of neighboring Moldova, means the unrest has spread across the breadth of southern and eastern Ukraine.

Western countries say Russian agents are directing the uprising and Moscow is stoking the violence with a campaign of propaganda, broadcast into Ukraine on Russian state channels, that depicts the government in Kiev as "fascists".

"Russia sometimes sounds as if it's refighting WW2. Fascism all over the place. Enemies everywhere. Ghosts of history mobilized," tweeted Swedish Foreign Minister Carl Bildt. (rt/ez)