* Ukraine beefs up forces, seeks Western aid against Russia
* War of words continues over Crimea secession referendum
* Ukrainian PM chides West over nuclear disarmament treaty
KIEV/SEVASTOPOL, March 11 (Reuters) - Ukraine's interim leaders established a new National Guard on Tuesday and appealed to the United States and Britain for assistance against what they called Russian aggression in Crimea under a post-Cold War treaty.
Blaming their ousted predecessors for the weakness of their own armed forces, acting ministers told parliament Ukraine had as few as 6,000 combat-ready infantry and that the air force was outnumbered nearly 100 to 1 by Moscow's superpower forces.
There was no let-up in the war of words, with the pro-Russian regional parliament in Crimea approving a declaration of independence that will take effect if people on the Black Sea peninsula vote to unite with Russia in a referendum on Sunday.
The national parliament in Kiev said it would dissolve the Crimean assembly if it did not cancel the plebiscite.
Viktor Yanukovich, whose overthrow last month after protests triggered the gravest crisis in Europe since the Cold War, insisted from his refuge in Russia that he was still Ukraine's legitimate president and commander of its armed forces.
Acting Prime Minister Arseny Yatseniuk, who will visit the White House and United Nations Security Council this week, said a 1994 treaty under which Ukraine agreed to give up its Soviet nuclear weapons obliged Russia to remove troops from Crimea and also obliged Western powers to defend Ukraine's sovereignty.
He said a failure to protect Ukraine would undermine efforts to persuade Iran or North Korea to forswear nuclear weapons as Kiev did 20 years ago. The terms of the Budapest Memorandum oblige Russia, Britain and the United States as guarantors to seek U.N. help for Ukraine if it faces attack by nuclear weapons.
Parliament passed a resolution calling on the United States and Britain, co-signatories with Russia of that treaty to "fulfill their obligations ... and take all possible diplomatic, political, economic and military measures urgently to end the aggression and preserve the independence, sovereignty and existing borders of Ukraine".
NATO powers - and the authorities in Kiev - have made clear they want to avoid a military escalation with Moscow, which has denied its troops are behind the takeover of Crimea 10 days ago by separatist forces - a denial ridiculed by other governments.
The European Union and United States have been preparing sanctions against Russia, though with some reluctance, especially in Europe, which values commercial ties with Moscow.
Direct diplomacy has stalled this week. U.S. Secretary of State John Kerry turned down an invitation to Moscow until Russia modifies its stance. Ukrainian premier Yatsneniuk said he had been unable to reach either Russian President Vladimir Putin or Prime Minister Dmitry Medvedev for the past five days.
Russia says the overthrow of Yanukovich was a coup backed by the West and that it has the right to defend the interests of the ethnic Russian majority in Crimea, a territory of two million that the Kremlin tranferred from Russia to Ukraine at a time when the collapse of the Soviet state was unthinkable.
NATO AWACs surveillance planes were beginning flights over Poland and Romania to monitor events in Ukraine and the U.S. navy was preparing for exercises in the Black Sea with NATO allies Bulgaria and Romania over the next few days.
Yatseniuk, who said he supported efforts to set up a "contact group" of major powers to resolve the crisis, accused Russia of seeking to undermine the world security system:
"This is not a two-sided conflict. These are actions by the Russian Federation aimed at undermining the system of global security," he told parliament.
Acting president Oleksander Turchinov said the National Security and Defence council had decided to raise a new National Guard among veterans. He accused Yanukovich of leaving the military in such a poor state that it had to be built "effectively from scratch".
The acting defence minister said Ukraine had not been prepared for military confrontration with Russia. Having mobilised its forces, he said the country had only 6,000 combat-ready infantry out of a nominal infantry force of 41,000 -compared to over 200,000 Russian troops on its eastern borders.
Turchinov warned against provoking Russian action, saying that would play into Moscow's hands. The National Guard, based on existing Interior Ministry forces, would "defend citizens from criminals and from internal or external aggression".
A partial mobilisation would begin of volunteers drawn from those with previous military experience, he said.
Yatseniuk said the government was doing all it could to finance pay and equipment for the armed forces, but that Kiev needed help from Western guarantors of its security.
Western powers have been careful to note that Ukraine, not being a member of NATO, has no automatic claim on the alliance to defend it. But Yatseniuk said the principles of its 1994 nuclear disarmament pact entitled it to expect assistance.
"What does the current military aggression of the Russian Federation on Ukrainian territory mean?" he said.
"It means that a country which voluntarily gave up nuclear weapons, rejected nuclear status and received guarantees from the world's leading countries is left defenceless and alone in the face of a nuclear state that is armed to the teeth.
"I say this to our Western partners: if you do not provide guarantees, which were signed in the Budapest Memorandum, then explain how you will persuade Iran or North Korea to give up their status as nuclear states." (Additional reporting by Natalia Zinets, Pavel Polityuk, Richard Balmforth and Ron Popeski in Kiev; Writing by Alastair Macdonald; Editing by Ron Popeski)