By Aleksandar Vasovic and Pavel Polityuk
SIMFEROPOL, Ukraine/KIEV (Reuters) - A Ukrainian serviceman was killed on Tuesday at a Ukrainian base that came under attack in Crimea's main town Simferopol, the first death on the peninsula from a military clash since the region came under Russian control three weeks ago.
As news spread of the death of the serviceman, in an assault on the base by unknown attackers, Ukraine's pro-Western prime minister denounced it as a "war crime" and called for international talks to prevent an escalation of the conflict.
Ukraine's defense ministry identified the attackers as Russian servicemen in uniform. An order was issued authorizing Ukrainian servicemen in Crimea to use weapons to protect their lives - in contrast to previous orders to exercise restraint.
Ukraine's acting president said Russia was annexing Crimea in actions reminiscent of Nazi Germany's takeover of Austria and Czechoslovakia's Sudetenland in the run-up to World War Two.
Two prominent political forces that took part in three months of demonstrations leading to the removal of Moscow-backed president Viktor Yanukovich called for Ukraine to break off diplomatic relations with its ex-Soviet master, Russia.
Ukrainian military spokesman Vladislav Seleznyov, speaking to Reuters from Crimea, said one serviceman at the Simferopol base had died of his wounds. A second man, a captain, was injured.
A defense ministry statement later said the dead serviceman had been shot and killed while manning a tower overlooking a vehicle pool at the base. It said the attackers had been wearing Russian military uniforms and were holding the base commander in a nearby building.
Interfax Ukraine news agency, quoting Crimean police, said a member of the pro-Russian "self-defense forces" was also killed in the incident. The report could not immediately be confirmed.
People in Crimea, home to Russia's Black Sea Fleet, voted overwhelmingly in a weekend referendum to join Russia, and a treaty to incorporate the region was signed in Moscow on Tuesday. Ukraine, the European Union and United States have denounced the referendum and proposed annexation as illegal.
Moscow and the region's pro-Russian leaders deny Russian forces are directly involved, saying "self-defense forces" are controlling the Black Sea peninsular region.
Prime Minister Arseny Yatseniuk, who came to power after Yanukovich's removal, said the conflict was moving "from a political one to a military one because of Russian soldiers.
"Today, Russian soldiers began shooting at Ukrainian servicemen and this is a war crime without any statute of limitations," he told a meeting at the defense ministry.
Yatseniuk said he had ordered Ukraine's defense minister to call a meeting with his counterparts from Britain, France, and Russia - signatories to a 1994 treaty guaranteeing Ukraine's borders - to "prevent an escalation of the conflict".
Acting President Oleksander Turchinov, referring to Hitler's Germany, told the meeting: "Putin today is copying the fascists from the last century by annexing the territory of another independent country recognized by the entire world."
The commander of Ukraine's navy told the meeting that five naval officers had been kidnapped in Crimea, with two later released. Ukraine's external intelligence service, quoted by Interfax, said one of its officers had been seized.
The political party led by boxing champion Vitaly Klitschko, who played a major role in three months of pro-Europe demonstrations in Kiev's Independence Square, said Ukraine had no choice but to break off ties with Moscow.
"Taking into account hostile actions of Russia's political leadership and the generally accepted practice of international relations, we insist on immediate termination of diplomatic relations between Ukraine and Russia," Klitschko's UDAR (Punch) party, a supporter of the government, said in a statement.
Oleh Tyahnibok, head of the right-wing Svoboda party, also a backer of the government, called for the severing of relations and the "creation of an anti-imperialist bloc of nations".
(Additional reporting by Mike Collett-White and Gabriela Baczynska in Simferopol and Alessandra Prentice and Natalia Zinets in Kiev; Writing by Ron Popeski; editing by David Stamp)