Ukraine's President Viktor Yanukovich (second from left) meets with opposition leaders Oleh Tyahnybok, Vitaly Klitschko and Arseny Yatsenyuk in Kiev on Wednesday. Andrei Mosienko / Presidential Press Service via Reuters

Ukraine's prime minister said on Wednesday that anti-government protests had brought "terrorists" onto the streets of Kiev, and he pledged to punish all "criminal action" when protesters confronted police near government headquarters.

Speaking to his Cabinet, Mykola Azarov took a tough line on the protesters, who on Wednesday massed anew in the hundreds.

Two activists were shot dead, the first fatalities in two months of anti-government protests.

Hours after the deaths were reported, police launched a fresh assault on the protesters in central Kiev, driving into their lines using tear gas and stun grenades.

The protesters fought back in intense clashes.

In words that appeared to foreshadow a police crackdown, Azarov said: "Terrorists from the 'Maidan' (Independence Square) seized dozens of people and beat them. I am officially stating that these are criminals who must answer for their actions."

Azarov accused opposition leaders of inciting "criminal action" by calling for anti-government protests, which he said destabilized the situation in Ukraine, a large former Soviet republic of 46 million people.

But in a move underlining US criticism of his government's treatment of the protesters, the US embassy in Kiev said it had revoked the visas of several Ukrainians linked to police violence against protesters in November and December.

It did not name the officials, but said it was considering further action against those responsible for the current violence.

The European Union called on Ukraine's government and opposition to engage in "a genuine dialogue".

"I strongly condemn the violent escalation of events in Kiev overnight leading to casualties. The reported deaths of several protesters are a source of extreme worry," EU foreign policy chief Catherine Ashton said in a statement.

The protesters have been out on the streets since November, angered by President Viktor Yanukovich's decision to shun a trade pact with the EU and instead accept financial aid from Russia to prop up Ukraine's ailing economy.

But events took a violent turn on Sunday after a mass rally called by the opposition to protest against sweeping new laws that ban any sort of anti-government protest.

Fifty people were detained overnight, and 29 of them were officially charged with taking part in mass unrest, police said. A total of 167 police officers have been injured. There was no immediate figure for civilians injured.

Azarov said earlier on Wednesday that police deployed on the streets did not possess firearms, and the Interior Ministry has denied that police have used guns during the crisis.

Wednesday's violence erupted as Ukraine marked National Unification Day - the day in 1919 that saw the merger of the two Ukranian nations - one a former part of Russia, the other a former part of the Austro-Hungarian Empire.

A 200-meter stretch of the city center close to government buildings and Parliament has been turned into a battle zone as hard-core protesters, ignoring opposition leaders' pleas for only a peaceful demonstration, have bombarded police with gasoline bombs and cobblestones. Riot police replied with rubber bullets, stun grenades and tear gas.

Reuters - AFP