* U.S. fourth-quarter GDP growth revised down to 2.4 pct
* Mounting tensions in Ukraine keep risk appetite in check
* U.S. says global surplus oil capacity rises
(Recasts to include U.S. GDP revision, updates comment, prices)
LONDON, Feb 28 (Reuters) - Oil dropped below $109 a barrel on Friday as tension in Ukraine dampened risk appetite and the United States revised its estimate for fourth quarter growth below analyst expectations.
The U.S. Commerce Department said that gross domestic product expanded at a 2.4 percent annual rate, down sharply from the 3.2 percent pace reported last month and just shy of analyst expectations, according to a Reuters poll.
A severe winter in the United States supported oil prices earlier this year, helping oil avoid the weakness of other risk assets such as base metals, as have supply losses in Libya and South Sudan.
But growing concern about the demand outlook for the United States and China, the world's largest and second-largest oil consumers, have weighed on prices. China's yuan looked set for its biggest daily loss on record on Friday.
"Overall we've just seen a little bit of concern with what's happening with global growth," said Simon Wardell, oil analyst at Global Insight.
"Prices have also been brought up on pretty cold weather lately, particularly in the United States. So I think the prospect of moving into the spring months where we get refinery maintenance has a dampening influence on prices."
Brent crude were down 34 cents to $108.62 a barrel by 1420 GMT, after dropping 56 cents in the previous session. U.S. oil was down 28 cents to $102.12.
The U.S. growth revision, released at 1330 GMT, pushed U.S. crude to a fresh intraday low of $101.80 before it narrowed losses.
Brent looks set to end the week down 1 percent, which would be its biggest drop in four weeks. U.S. crude is set to end the week slightly lower, ending six straight weeks of gains - the longest period of weekly rises in a year.
Rising tension in Ukraine has also contributed to a broader emerging markets sell-off that has weighed on risk assets such as oil.
"Ukraine might be playing into wider concerns over global growth and the economy. I'm not sure it's having a huge impact right now but obviously we'll see how things move in the coming days," Wardell said.
Armed men took control of two airports in the Crimea region on Friday in what Ukraine's government described as an invasion and occupation by Russian forces.
"The current situation is more bearish than bullish for oil prices as oil supply has not been affected so far and the dollar is strengthening against the Ukrainian currency (the hryvnia) and the euro as risk appetite is hit," said oil brokers PVM in a report.
Oil supplies appear to be more plentiful. Iranian exports have increased this year.
Global spare production capacity inched higher in January and February, the U.S. government's Energy Information Administration said on Thursday.
(Additional reporting by Alex Lawler in London and Manash Goswami in Singapore; editing by Jason Neely)