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.

Russian President Vladimir Putin has declared a relatively modest income of just over $100,000 for last year -- the lowest of all of his staff in the Kremlin, official figures showed Friday.

Putin earned about 3.7 million rubles ($104,000, 75,000 euros) and owns three Russian-made cars, a 77 square-meter flat, a garage and a plot of land, according to a declaration of assets published on the Kremlin's website.

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.

Masked gunmen have shot dead an Islamic teacher after bursting into his home in Russia's Dagestan, in the latest such killing in the volatile region, investigators said on Friday.

"At around 11:00 pm on Thursday, unidentified attackers in masks entered the house of 32-year-old Magomed Zakaryayev, the teacher at the madrasa in the village of Nechayevka and fired several shots," the Investigative Committee, which probes major cases in the country, said in a statement.

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.

A group of Russian lawmakers have formally requested prosecutors to probe former Soviet leader Mikhail Gorbachev for treason over the breakup of the Soviet Union, one lawmaker said Thursday.

Ivan Nikitchuk, a lawmaker with the Communist party, said that recent events and the crisis in Ukraine in particular have led several MPs, including some from the ruling party, to ask Prosecutor General Yury Chaika to probe Gorbachev, 83.

President Vladimir Putin on Thursday sent a letter to leaders of 18 European countries, warning them Russia could cut gas supplies to Ukraine, but suggesting that Moscow and Brussels work together to help restore the ex-Soviet country's battered economy.

If Ukraine does not settle its 1.6 billion euro ($2.2 billion) energy bill, natural gas giant Gazprom would be "compelled to switch over to advance payment for gas deliveries, and in the event of further violation of the conditions of payment, will completely or partially cease gas deliveries," Putin wrote in the letter.