* First Crimea death in clashes since Russian takeover
* Defence minister told to hold talks on preventing escalation (Combines stories, adds death of serviceman)
KIEV, March 18 (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 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 that led 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 by telephone from Crimea, said one serviceman at the Simferopol base had died of his wounds. A second man, a captain, was injured.
Seleznyov said the dead serviceman was responsible for overseeing a vehicle pool at the base. He said the attackers had told the Ukrainian servicemen that they were under arrest and their documents were confiscated.
It was unclear, Seleznyov said, who had staged the assault. He described the attackers as "unknown forces, fully equipped and their faces covered".
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-defence 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 defence ministry.
Yatseniuk said he had ordered Ukraine's defence 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 recognised by the entire world."
The political party led by former 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 Alessandra Prentice and Natalia Zinets; Writing by Ron Popeski; Editing by Mark Heinrich)