By Tom Wilson and Karen Brettell

LONDON/NEW YORK (Reuters) – Bitcoin tumbled to its lowest level in six months on Monday as fears of a Russian attack on Ukraine saw riskier assets worldwide extend their sell-off.

The largest cryptocurrency was last down 5.4% at $34,310 after earlier dropping to $32,951, its lowest price since July 23, taking losses from its all-time high of $69,000 hit in November past 50%.

NATO said on Monday it was putting forces on standby and reinforcing Eastern Europe with more ships and fighter jets, in what Russia denounced as an escalation of tensions over Ukraine.

Fears of a conflict pummelled shares across the world while bolstering the dollar and oil.

Nerves over the Federal Reserve’s two-day policy meeting, starting on Tuesday, added to the mix, with the U.S. central bank expected to confirm it will soon start draining the pool of liquidity that has supercharged growth stocks.

“The story is really how aggressive is the tightening,” said Marc Chandler, chief market strategist at Bannockburn Global Forex in New York, noting that a Fed move to reduce the size of its balance sheet will tighten conditions along with higher interest rates.

Crypto currencies are falling along with other risk assets as “people are pulling back on risk broadly,” he added.

Smaller cryptocurrencies, which tend to move in tandem with bitcoin, also slumped. Ether, the second-largest digital coin, was last down 11% at $2,257 after earlier reaching $2,160, its lowest level since July 27.

More than $400 million in crypto assets has been liquidated in the past 24 hours, according to data from Coinglass, with Bitcoin trades accounting for $141 million of that total.

“Bitcoin will face headwinds going back up until the macroeconomic conditions change,” said Mark Elenowitz, president of Horizon, a firm that services securities exchanges.

“Generally speaking, when rates are hiked, we could see more sell-offs of seemingly risk-on assets like bitcoin.”

U.S.-listed cryptocurrency miners Riot Blockchain, Marathon Digital and Bit Digital slumped between 9% and 17% on Monday, while crypto exchange Coinbase Global dropped 13.3%.

(Additional reporting by Medha Singh in Bengaluru; Editing by Rachel Armstrong, Mark Potter and Paul Simao)