KIEV, April 21 - Ukraine is planning an operation involving tens of thousands of police to guard against any attack by separatists or Russian agents during World War Two commemorations next month, security chiefs said on Tuesday, Reuters reported.

Tension is mounting in the capital and other cities amid an increase in rebel attacks in the east. Kiev said one Ukrainian serviceman had been killed in the past 24 hours, in an attack near the airport in Donetsk. The airport fell to the rebels earlier this year.

The killings in Kiev of two pro-Russian activists, a journalist, by what appeared to be professional hitmen, have further driven up tension in the run-up to May 8-9 celebrations of victory in 1945, which traditionally bring thousands of people on to the streets.

Ukraine, along with most European Union members and the United States, is boycotting festivities in Moscow marking 70 years since the allied victory over Nazi Germany, because of Moscow's role in the conflict in eastern Ukraine in which more than 6,100 people have been killed.

But it will hold victory celebrations of its own in Kiev and other cities.

"We cannot trust the word of Russia and their terrorists at all. We must be ready to give a clear, appropriate and strong reply to protect people on the streets, provide warnings of terrorist attacks and bring those guilty of crimes to justice," Prime Minister Arseny Yatseniuk told security chiefs.

Calling for heightened security measures, particularly in large cities, Yatseniuk said Russia was spending a lot of money on financing networks to stir up trouble.

"We are fighting a state which has planned dozens of terrorist acts and we must do all we can to head them off. Social, political and ideological detribalization -- that's the aim of Russia." he said.

Interior Minister Arsen Avakov said 10,000 guards were ensuring security at 3,300 of the most sensitive facilities in the country, including nuclear power stations.
A total of 20,000 extra security and police would be drafted in for the May festivities. "We are ready to ensure calm," he said.

Defense Minister Stepan Poltorak, speaking of the situation in the east, said "terrorist threats" were growing and protection of arms and military equipment arsenals and depots would be stepped up. (rt/ez)