Baltic leaders said on Saturday they will back further sanctions against Russia at a European Union summit next week if the conflict continues to escalate in eastern Ukraine where Kiev is trying to quell a pro-Russian insurgency.

Hours after the start of a cease-fire on Friday, separatists attacked Ukrainian posts on the border with Russia and a military base, and tried to storm an air force base, Ukrainian security forces said.

Earlier on Friday, the leaders of the United States, France and Germany agreed Russia risks new sanctions if it fails to take immediate steps to defuse tensions on the Ukraine border.

EU leaders are expected to discuss the issue at a summit in Brussels next week.

Latvia, Lithuania and Estonia, which all broke away from the Soviet Union in 1990 and1991, had already called for tougher EU sanctions against Moscow after its annexation of Crimea.

"If there will be no deescalation of the situation, then Latvia will support a third bloc of sanctions," Latvian Prime Minister Laimdota Straujuma told a news conference after meeting with her Baltic counterparts and European Commission President Jose Manuel Barroso in the Estonian capital.

Lithuanian Prime Minister Algirdas Butkevicius and Estonian Prime Minister Taavi Roivas endorsed her statement.

So far, the EU has imposed limited measures such as asset freezes and travel bans on people and companies in Russia and Ukraine. The 28-member bloc has discussed other steps against Russia including restrictions ranging from bans on luxury goods imports to an oil and gas. There has been no consensus on how to proceed.

States with close trade and energy ties with Russia, such as Italy, Greece and Germany, worry about the potential impact on their economies of stricter sanctions while others, such as Cyprus and Austria, have close financial links.

Concerns over Europe's gas supplies are a major consideration in the decision around sanctions as EU consumers get about a third of their gas needs from Russia, about half of it through pipelines that cross Ukraine.

Barroso at the news conference urged EU member states to reach a common stance on the issue of further sanctions.

"I am pleading for a common position of the member states," he said.