President Vladimir Putin's spokesman said Friday Ukraine's offensive against pro-Russian rebels in the east dealt a final blow to a peace deal on the crisis, adding a Moscow envoy had been dispatched there to help negotiate the release of kidnapped OSCE monitors.

"While Russia is making efforts to de-escalate and settle the conflict, the Kiev regime moved combat air forces against peaceful settlements, began a reprisal raid, essentially finishing off the last hope for the feasibility of the Geneva accords," Dmitry Peskov was quoted as saying by Russian news agencies.

Ukraine on Friday launched a military assault on the flashpoint town of Slavyansk, raising the stakes in the showdown with Russia, which has vowed "catastrophic consequences" if Kiev stepped up operations.

Insurgents shot down two army helicopters, killing two servicemen, including a pilot, as the army tightened its noose around the rebel-held town of 160,000 people.

Some 60,000 people waving Russian flags and brandishing posters of President Vladimir Putin took to the streets Thursday in Simferopol, capital of the Crimean peninsula annexed by Moscow in March.

The marchers carried banners reading: "We are Russia" and "Putin is our president", said an AFP reporter on the scene who estimated the size of the crowd.

A crowd of some 300 pro-Russian militants attacked the prosecutor's office in the eastern Ukrainian city of Donetsk on Thursday, Agence France Presse reporters on the scene said.

The mob hurled rocks and Molotov cocktails at around 100 riot police defending the building, who responded with stun grenades and tear gas, in the latest unrest to hit the crisis-wracked eastern part of Ukraine.

Dutch police on Thursday arrested around 30 Greenpeace activists, including the captain of the lobby group's flagship Rainbow Warrior, as they tried to stop a Russian tanker delivering Arctic oil from docking.

"The captain has been arrested and the ship is being taken elsewhere else," Rotterdam police spokesman Roland Eckers told AFP of the Rainbow Warrior.

Some 100,000 workers joined a May Day parade on Moscow's Red Square Thursday for the first time since the 1991 Soviet breakup, as the takeover of Crimea boosts President Vladimir Putin's standing amid a surge of patriotism.

A huge column of demonstrators waving Russian flags and balloons marched through the iconic square near the Kremlin walls as trade union leaders addressed them from the podium on International Labor Day.

Ukraine has arrested the Russian defense attache in Kiev and decided to bring back military conscription with immediate effect to deal with a spreading pro-Moscow insurgency in its east.

According to a decree issued Thursday by interim president Oleksandr Turchynov, the country was bringing back military conscription "given the deteriorating situation in the east and the south ... the rising force of armed pro-Russian units and the taking of public administration buildings ... which threaten territorial integrity."

China and Russia will carry out joint naval exercises in the East China Sea later this month, state media reported Thursday, with China and Japan embroiled in a territorial dispute in the area.

The drills, to take place off Shanghai, are scheduled for late in the month, the official Xinhua news agency said in a brief report, citing China's defense ministry.

A leader of the self-declared Donetsk People's Republic said Wednesday the eastern Ukrainian region will not take part in presidential polls as he traveled to Russia on an apparent mission to seek support.

"Presidential polls have been called by the illegitimate authorities and therefore they are illegitimate," Denis Pushilin, one of the leaders of the self-declared Donetsk Republic, told reporters in Moscow.

The European Union is considering beefing up sanctions against Russia by targeting President Vladimir Putin's inner circle but some member states are "very reluctant," EU diplomats said Wednesday.

A proposal seen by Agence France Presse that was circulated to member states by Britain this week calls on the bloc to increase pressure on Putin by sanctioning his "inner circle" or "money men" in response to the deepening crisis in Ukraine.