* US dollar posts best one-day gain in a month, hits two-week highs
* U.S. warns Russia it would be 'grave mistake' to intervene militarily in Ukraine
* Aussie dollar eyes capital spending data
By Ian Chua
SYDNEY, Feb 27 (Reuters) - The U.S. dollar held near two-week highs against a basket of major currencies early on Thursday, after rallying overnight even as U.S. Treasury yields fell further.
Traders said the move was a bit of a puzzle but pointed to heightened tensions in Ukraine and an unexpectedly upbeat report on U.S. new home sales as supporting the greenback.
The dollar index climbed 0.4 percent, its biggest one-day rise in about a month, to as high as 80.524. It has since retreated slightly to 80.415.
The greenback drifted up to 102.40 yen, but remained well within its previous session's range of 102.01/102.63, while the euro traded at $1.3683, near a two-week trough of $1.3661 plumbed overnight.
Underpinning the dollar, emerging market currencies came under renewed pressure led by a fall in Ukraine's hryvnia after the central bank abandoned a managed exchange rate policy in favour of a flexible currency.
But Barclays Capital analyst, Eldar Vakhitov, viewed that as a positive move.
"We see the abandonment of the peg as positive news because allowing for greater exchange rate flexibility has been one of the requirements outlined by the IMF," Vakhitov wrote in a note to clients.
"The step has resultant costs however, not least of which is that it could aggravate internal FX demand and may even accelerate deposit flight pressure," he said.
Raising tensions in the region, President Vladimir Putin ordered 150,000 Russian troops to be ready for war games near Ukraine, prompting the United States to warn Russia that it would be a 'grave mistake' to intervene militarily.
Protest leaders in Ukraine, a former Soviet republic that Putin has called a "brother nation", want to form a new government following the overthrow of pro-Russia President Viktor Yanukovich.
Despite the risk of greater instability in the region, there was no widespread panic in markets with Wall Street ending nearly flat, while safe-haven currencies like the Swiss franc and yen were mixed.
Renewed demand for the U.S. dollar came at the expense of commodity currencies such as the Australian dollar, which quickly dipped back below 90 U.S. cents to last trade at $0.8965 .
The Aussie's immediate fortunes will depend on capital spending data due at 0030 GMT. Any upside surprise in spending plans could help put a floor under the currency.
Federal Reserve Chair Janet Yellen's weather-delayed testimony before the Senate Banking Committee later on Thursday will also be closely watched.
"It would be a huge surprise if she amended the prepared testimony, but there should be plenty of interest in the lengthy Q&A session in front of the Senate Banking Committee," said Sean Callow, currency strategist at Westpac in Sydney.
"One key line of inquiry should be her view on how much of U.S. data weakness lately can be blamed on the weather."