Oren Dorell

USA TODAY

The Ukrainian army said Thursday it destroyed two bases for pro-Russian militants in overnight operations after the start of European-brokered talks that have faltered because the two sides will not meet.

President Oleksandr Turchynov told the parliament in the capital of Kiev that government forces attacked a militant base in Slovyansk and another in nearby Kramatorsk in eastern Ukraine. Ukraine's Defense Ministry said there were no casualties.

In London, Secretary of State John Kerry said the United States hopes Russia will play a constructive role in Ukraine's presidential election May 25. He warned that if Moscow or its backers in Ukraine disrupt the voting, the United States and European Union will impose heavier economic sanctions.

"I'm not going to get into announcing today what the sanctions are," Kerry said after a meeting in London with diplomats from Britain, France, Germany and Italy. "If they have to go into effect, they will have an impact."

"Let me emphasize, our hope is not to do this," Kerry said. "Our hope is not that we have to go to the next stage. I say to the Russians and everybody, our hope is to de-escalate."

A senior State Department official told the Associated Press that the sanctions would target vulnerabilities in Russia's business, banking, mining, energy, defense and other sectors. The official, who was not authorized to speak about the discussions in London, spoke on condition of anonymity.

Militants have seized government buildings across eastern Ukraine and declared two regions independent after a referendum Sunday that was dismissed as a sham by the Ukrainian government and the West.

The Organization for Security and Cooperation in Europe, a trans-Atlantic security group, put forward a "road map" calling for national dialogue as a first step toward resolving the escalating tensions. The first round of talks in Kiev produced no visible result as the government refuses to sit down with representatives of the insurgents.

Russia is accused by the West of backing the militants. Thursday, Russia said Ukraine must pay in advance for Russian gas supplies, starting June 1.

Russian President Vladimir Putin said in a letter to European leaders that Ukraine's debt for Russian gas supplies has reached $3.5 billion, and because of its refusal to pay Moscow, it will have to switch to prepaid gas deliveries.

The Russian president first warned of the move in April in a letter to European leaders, whose nations are customers of Russia's state-controlled Gazprom natural gas giant. Ukraine serves as a major conduit for Russian gas supplies to Europe, and pricing disputes have led to shutdowns.

Contributing: The Associated Press