“Attempts at military conflict in Ukraine will lead to a military conflict in Europe,” Yatsenyuk told a cabinet meeting broadcast live and translated by Reuters. “The world has not yet forgotten World War II, but Russia already wants to start World War III.”

Russian President Vladimir Putin, meanwhile, warned that any violence against pro-Russian militia in Ukraine would necessitate “consequences.”

“[The confrontations] are just a punitive operation and will, of course, incur consequences for the people making these decisions, including on our interstate relations,” Putin said in a televised meeting with regional media on Friday.

Russia and Ukraine reached an agreement last week in Geneva that called on all parties in the country to lay down arms and vacate public buildings. Pro-Russian militias have been occupying government buildings in more than 10 cities in eastern Ukraine, and the nationalist Right Sector movement is still in control of two public buildings in Kyiv.

But tensions have not eased, and on Friday there were scattered reports of more violence. Ukraine’s Defense Ministry said a grenade fired from a launcher caused an explosion in a helicopter at an airfield outside the eastern city of Kramatorsk. Deputy Minister Vasyl Krutov said the pilot was injured.

Ukrainian forces clashed with pro-Russian armed groups as they closed in on the city of Slovyansk on Thursday, seizing checkpoints and setting up roadblocks as helicopters circled overhead. On Friday a bus carrying international observers in the city from the Organization for Security and Cooperation in Europe was seized. The Ukrainian Ministry of Interior blamed the capture on pro-Russian armed groups.

The separatist self-declared mayor of Slovyansk told Reuters the mediators were being held because they were believed to have a spy among them from the pro-Western government in Kyiv.

"People who come here as observers bringing with them a real spy: It's not appropriate," Vyacheslav Ponomaryov said in front of a security service building occupied by separatists where the Ukrainian government said the observers were being detained.

In southeastern Ukraine, seven people were injured by a blast at a checkpoint set up by local authorities and pro-Ukraine activists outside the Black Sea port of Odessa. Local police spokesman Volodymyr Shablienko said unknown men threw a grenade at the checkpoint.

In a message posted to Facebook, Interior Minister Arsen Avakov said the mission to take back buildings occupied by pro-Russian armed groups was ongoing. “ATO [anti-terrorist operation] continues. Terrorists should beware around the clock. Civilians have nothing to fear,” he said.

Serhiy Pashinskiy, the Ukrainian president’s chief of staff, reiterated his remarks, saying they were attempting to encircle the city of Slovyansk, where the five rebels were killed on Thursday and where pro-Russian rebels have been particularly active.

Meanwhile, Lavrov on Friday accused the West of plotting to control Ukraine and said the pro-Russian armed groups in the southeast would lay down their arms only if the Ukrainian government clears out the Maidan protest camp in Kyiv. “The West wants — and this is how it all began — to seize control of Ukraine because of their own political ambitions, not in the interests of the Ukrainian people,” he said.