Turkey on Saturday dismissed as "out of the question" claims from Russia that it had allowed U.S. warships to stay longer in the Black Sea than permitted under international law.

Russia has complained that U.S. warships have remained in the Black Sea longer than the 21 days allowed by an international treaty, amid ongoing tensions between Moscow and the West.

The leader of France's far-right National Front voiced her support for the federalization of Ukraine during a visit to Moscow Saturday, Russian news agencies reported.

Marine Le Pen, who is also a European Parliament lawmaker, told the speaker of Russia's lower house Sergei Naryshkin she backed Moscow's call for Ukraine to devolve greater powers to its regions.

Ukraine accused Moscow of "aggression" on Saturday after Kalashnikov-wielding gunmen seized two security building in its restive eastern rust belt amid spreading protests demanding the Russified region join Kremlin rule.

The coordinated attacks and a series of gunfights between militants and police in two eastern towns underscored the volatility of the crisis ahead of first direct talks between EU and U.S. diplomats and their Moscow and Kiev counterparts in Geneva on Thursday.

Pro-Russian separatists retained control of key government buildings in eastern Ukraine Saturday despite a vow by the country's premier to cede more power to the regions, amid rising tensions between Russia and the West.

Prime Minister Arseniy Yatsenyuk's promise came during a visit Friday to the coal mining region of Donetsk as militants armed with Kalashnikovs barricaded themselves inside the local government building and demanded a referendum on joining Russia.

Planned four-way talks on the crisis in Ukraine between the U.S., EU, Russia and Ukrainian government will take place in Geneva on April 17, a U.S. official confirmed Friday, as Washington unveiled sanctions against a number of Crimea breakaway leaders.

U.S. Secretary of State John Kerry will attend the meeting to "continue efforts to de-escalate the situation in Ukraine and find a diplomatic path forward," State Department spokeswoman Jen Psaki said.

Russia's Foreign Minister Sergei Lavrov on Friday denied that Moscow had sent any soldiers or security agents to Ukraine's eastern regions where a separatist movement has demanded independence from Kiev, warning that European stability was being threatened by rising anti-Russian sentiment over Ukraine.

"We are accused of having security agents there. They are not there," Lavrov was quoted by the RIA Novosti news agency as saying on state television.

Seven miners were killed on Friday when a blast tore through a mine in the eastern Ukrainian region of Donetsk, officials said, blaming the accident on safety rule violations.

The explosion occurred in the morning hours at a depth of 1,300 meters (4,250 feet) with 52 miners in the shaft, the local prosecutor's office said

U.S. President Barack Obama told German Chancellor Angela Merkel on Thursday that the United States and its allies should prepare new sanctions against Russia if it escalates the crisis over Ukraine.

"The president underscored the need for the United States, European Union and other global partners to be prepared to meet further Russian escalation with additional sanctions," the White House said in a statement about the phone call.

Kiev on Thursday charged that Russian troops were laying anti-personnel mines in Ukraine and had grabbed a Ukrainian stockpile of landmines used for training.

Ukraine "is strongly concerned about the use of anti-personnel mines by the Russian Armed Forces in several parts of the Ukrainian territory," a Ukrainian delegation said at a meeting on landmines in Geneva.

NATO should redeploy its forces in eastern Europe and make the Black Sea region a priority after Russia's annexation of Crimea, Romania's foreign minister urged in an interview Thursday with Agence France Presse.

"Romania has concrete expectations of a redeployment and an eastward repositioning of NATO's naval, air and ground forces," Titus Corlatean said.