Many emerging Asian currencies started the day firmer after solid US housing data and growing hopes that Spain will ask for a bailout next month boosted optimism on the global economy.
However, they turned slightly weaker or gave up most of their initial gains as the Singapore dollar fell, with the Monetary Authority of Singapore (MAS) seen buying the US dollar to slow down gains in this year's best performing Asian currency.
Some dealers in Seoul also suspected South Korea's foreign exchange authorities of selling the won, which hit hit a near six-month high versus the yen. Major South Korean companies compete with Japanese firms in key overseas markets.
The Philippine peso eased as investors took profit amid caution that the central bank is unlikely to allow the local unit strengthen past 41.20 per dollar.
"Asian countries may have no choice but to intervene in currency markets or cut rates as recent economic data have been pretty bad, although a rebound in China's economy may help," said Yuna Park, a currency and bond analyst at Dongbu Securities in Seoul.
On Wednesday, Thailand's central bank unexpectedly slashed its main interest rate to shore up domestic demand as the economy might be weaker than forecast in 2013 due to a gloomy global outlook.
Intervention by regional authorities came even as China's economy showed signs of improvement, dealers said.
The world's second-largest market slowed for a seventh straight quarter in July-September, but industrial production, retail sales and investment data were all slightly better than expected.
Those indicators eased worries about sluggish China growth, suggesting emerging Asian currencies were eventually likely to rise further, dealers and analysts said.
The economic concerns, along with the euro zone's debt crisis, have been one of major hurdles to appreciation in regional currencies.
"China data could be signaling a bottoming out for China and the risk rally is still in play," said a European bank dealer in Manila.
SINGAPORE DOLLAR
The Singapore dollar edged lower as the MAS was spotted intervening, dealers said.
The Singapore dollar's nominal effective exchange rate (NEER) earlier in the day was around 1.4 percent above its perceived parity with the extreme strong end of the NEER currently estimated at around 1.2170 per US dollar, OCBC Bank said in a note.
RINGGIT
The ringgit advanced, but it gave up most of the initial rise as a weaker Singapore dollar caused investors to cover dollar-short positions.
Dollar demand linked to daily fixing also put pressure on the Malaysian currency.
But the ringgit is still holding upside momentum as worries about a global slowdown ease.
"Investors bought back dollars, but only to sell around 3.05 again. I will sell dollars or keep short positions as long as the euro stays above 1.3," said a Malaysian bank dealer in Kuala Lumpur.
The euro stood at $1.3104.
WON
The won edged up against the dollar, but its upside was capped as caution over dollar purchases by the foreign exchange authorities prompted investors to take profits.
Some traders suspected intervention as the local currency cut gains, even though Chinese data eased worries about a slowdown in South Korea's top overseas market.
"Dollar bids elevated even after the good Chinese data," said a foreign bank dealer in Seoul.
The won strengthened 0.4 percent to 13.9344 against the yen, its strongest since April 27.
Major South Korean companies such as Hyundai Motor Co compete with Japanese peers including Toyota Motor Corp in export markets.
PHILIPPINE PESO
The Philippine peso slid as investors covered dollar-short positions, with traders wary of intervention.
Standard & Poor's also said the Philippines, along with Indonesia, has some weakness to address before it is likely to break into the investment-grade rating category.
Some dealers looked to buy the peso on dips, given the country's stronger economic fundamentals.
"Still, there is pretty much the chance for the upgrade, although I don't think people were expecting the upgrade to come this year," said a foreign bank dealer in Manila.
The dealer said he would buy the peso when it approaches near 41.40 per dollar.