A Ukrainian military helicopter exploded at a base near the eastern town of Kramatorsk on Friday after being hit by a rocket-propelled grenade, officials in Kiev said. The helicopter was hit in the fuel tank and exploded.

“An army Mi-8 helicopter has exploded at the Kramatorsk aerodrome,” the defence ministry in Kiev said in a statement.

The chopper was on the ground when it was targeted and that the pilot managed to escape but sustained injuries.

The development came as the International Criminal Court’s (ICC) chief prosecutor opened a initial probe into crimes committed before and during the fall of Ukraine’s ousted president Viktor Yanukovych.

“The prosecutor of the ICC, Fatou Bensouda, has decided to open a preliminary investigation into the situation in the Ukraine to establish whether… the criteria for opening a (full) investigation are met,” a statement said.

“The prosecutor shall consider issues of jurisdiction, admissibility and the interests of justice.

“If there was a reasonable basis for an investigation, it is then up to her (Bensouda) to ask the court’s judges for authorisation.”

Arseniy Yatsenyuk, the country’s prime minister, accused Russia of wanting to start “a third world war” and called for for international help against “Russian aggression” after it emerged that Russian troops were on exercises less than a mile from the border.

“The aggressive efforts of the Russian military on Ukraine’s soil will lead to a conflict on European soil,” he said in a televised cabinet meeting. “Russia’s support for the terrorists in Ukraine constitutes an international crime and we call on the international community to unite against the Russian aggression.”

John Kerry warned that President Vladimir Putin is running out of time to comply with an accord to ease tensions in Ukraine, as Russia’s military began new exercises on the countries’ border.

Kerry accused Russia of using the “barrel of a gun and the force of a mob” to impose its will on Ukraine. He said it’s failed to live up to commitments it made a week ago in Geneva to de-escalate the situation, and continued lack of cooperation would bring consequences. President Barack Obama plans to call European leaders to discuss sanctions today.

“If Russia continues in this direction, it will not just be a grave mistake, it will be an expensive mistake,” Kerry said at the State Department in Washington yesterday.

Latvia welcomed American troops on its soil, part of a US force of 600 sent to the region to reassure the Baltic states amid concern over Russia’s actions in Ukraine.

“Today is a special day because this morning I met the heads of the armed forces at the Adazi military base and greeted the US military unit that arrived this morning for military training,” Laimdota Straujuma, the prime minister said.

Some 150 troops from the 173rd Airborne Brigade will be based at the Adazi base near capital Riga until at least the end of the year, according to the Latvian defence ministry.

Another company of soldiers arrived in Poland on Wednesday, while around 150 others are each expected in Lithuania on Saturday and Estonia early next week.

Kerry spoke hours after Putin warned Ukraine against continuing an anti-separatist offensive that killed five rebels. An accord on de-escalation signed April 17 by Ukraine, Russia, the European Union and the US is on the brink of collapse.

Ukrainian special forces taking position in the eastern Ukrainian city of Slavyansk on Thursday (AFP)

Ukrainian special forces launched a second phase of their operation in the east of the country on Friday by mounting a full blockade of the rebel-held city of Slavyansk, an official on the presidential staff said.

“Ukrainian special forces units began the second stage, which consists of our taking the decision to blockade the city of Slavyansk completely and give no opportunity to bring in reinforcements,” Serhiy Pashynsky was quoted as telling reporters by a presidential spokesman.