Mounting separatist tensions in Ukraine's Russian-speaking Crimea peninsula have reached dangerous levels and could lead to fresh domestic turmoil or even spark an outright invasion by giant neighbor Russia, analysts say.

On Thursday, pro-Moscow gunmen seized control of the parliament and government headquarters in the regional capital Simferopol, sparking warnings from Ukraine's interim leader that any troop movements at Russia's Black Sea fleet -- based in Crimea -- would be seen as "military aggression".

Russian President Vladimir Putin on Wednesday ordered a snap check of the battle-readiness of the armed forces in the west and center of the country, including the area bordering Ukraine, as U.S. Secretary of State John Kerry warned that any Russian military moves in Ukraine would be a "grave mistake."

"The commander-in-chief has set the task of checking the capability of the armed forces to deal with crisis situations posing a threat to the military security of the country," said Defense Minister Sergei Shoigu, quoted by the Interfax news agency.

More than 400 Russian protesters faced court hearings on Tuesday a day after police detained them at a central Moscow rally, the largest such wave of arrests in nearly two years.

The demonstrators had gathered near Red Square late on Monday in support of activists jailed earlier in the day for staging "mass riots" in May 2012, a key case seen as a symbol of the harsh crackdown on dissent under President Vladimir Putin's latest term.

Francois Hollande told Russia's Vladimir Putin on Monday Ukraine must have a "peaceful transition", saying it was vital to ensure the country remained unified, the French president's office said.

Hollande also said the formation of a new government in Ukraine must lead to a presidential election and vital reforms, in a telephone conversation with the Russian president.

Russian police detained hundreds of protesters Monday outside a court that sentenced seven activists to prison terms of up to four years for demonstrations against Vladimir Putin's third presidential inauguration.

The Moscow court sentenced the seven defendants to penal colony terms of between two and a half and four years for what the judge ruled to be "mass riots" during the 2012 demonstrations, a high-profile case that has become a symbol of the harsh crackdown on opposition protests since Putin returned to the Kremlin.

German Chancellor Angela Merkel and Russian President Vladimir Putin agreed Sunday that Ukraine's "territorial integrity" must be ensured, the German government spokesman said, as the U.S. said it was in no one's interest to see Ukraine break apart.

Merkel spoke by phone with Putin and "both agreed that Ukraine must quickly get a government capable of acting and its territorial integrity must be preserved," Steffen Seibert said.

Two hundred people were arrested Friday near a Moscow court as it convicted eight defendants of mass riots and attacking police at a protest against President Vladimir Putin, a case widely seen as a crackdown on dissent.

The eight defendants were found guilty of participating in mass riots and hitting policemen at an opposition rally the day before Putin returned to the Kremlin for a third term in 2012.

German Chancellor Angela Merkel and U.S. President Barack Obama joined with Russian President Vladimir Putin on Thursday to call for an urgent political solution to the deadly crisis gripping Ukraine.

"The chancellor and the presidents have reached an agreement to call for a political solution to the crisis in Ukraine as quickly as possible and for an end to the bloodbath," the German government said in a statement released after phone calls between the Western leaders and Putin, who have been at odds over Ukraine's future.

Moscow said Monday it would release two billion dollars this week from a vital aid package to Ukraine it had effectively frozen when protests escalated last month.

Asked about the next tranche of the $15 billion package, Finance Minister Anton Siluanov told Russian news agencies: "This week, two billion."

Iran's ambassador to Moscow on Monday said Russia could build the Islamic republic a second nuclear power reactor in exchange for massive oil shipments that have raised grave concern in Washington.

Ambassador Mehdi Sanaei said the two close trading partners have been negotiating Iran's delivery of hundreds of thousands of barrels of oil a day since a meeting at a regional summit in September between Russian President Vladimir Putin and his counterpart Hassan Rouhani.