Nikkei Clings Onto Gains After BOJ Stands Pat

Confidence returned to most Asian stock markets on Wednesday after investors were reassured that global central banks would not pare down their economic stimulus programs.

Japan's benchmark Nikkei index hit a fresh five-and-a-half-year high, little changed after the Bank of Japan (BOJ) kept monetary policy steady as it concluded a two-day meeting.

Elsewhere, the Kospi closed within sight of the key 2,000 level, but Australian stocks were weighed down by weak commodity prices and the Shanghai Composite snapped a five-day winning streak.

Central Bank Watch

The BOJ raised its assessment of the domestic economy but made no reference to the Japanese government bond market (JGBs) in its policy statement. Investors were hoping Governor Haruhiko Kuroda would fine-tune market operations to cool spiking yields.

(Read More: Why BOJ Was Right Not to Talk Up Bonds)

Comments by Fed officials overnight dampened speculation that the U.S central bank will taper its bond-buying program. Attention now turns to Fed Chairman Ben Bernanke's testimony to the Senate on Wednesday for final confirmation.

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Nikkei Crosses 15,500

An 6 percent gain in Sony shares led Japanese stocks higher on hopes that the electronics giant will back Daniel Loeb's call to spin off its entertainment division at its corporate strategy update later on Wednesday.

(Read More: Is Sony 'Un-Japanese' Enough to Change?)

Sojitz surged 12 percent after the Nikkei newspaper reported that the conglomerate will post a 30 percent rise in pretax profit for the year ending March 2015.

Japan's April trade figures revealed the negative impact of the yen's weakness as higher import costs led to a trade deficit for the tenth straight month. The dismal data highlights the depth of the nation's weak economic base.

"Japan has gone all in on Abenomics, as in a poker game - You can think of it as all in or double or nothing. There's a lot of excitement and hope that things are different but there's not a lot of confidence about the longer-term outlook," said Damian Lillicrap, head of investment strategy at Q-Super.

China Down 0.1%

The mainland's benchmark index pared earlier gains but still remains set to post a 6 percent monthly gain for May.

Power producers were sold-off after Beijing proposed to ban coal imports. Huadian Power lost 5 percent, Huaneng Power tumbled 6 percent and GD Power Development fell 4 percent.

Investors are looking ahead to HSBC's Flash PMI data, due for release on Thursday, for a check-up on the mainland's manufacturing sector. The figure has been weaker in recent months and any further downturn may end the market's recent bullish momentum.

Australia Dips

The S&P ASX 200 reversed initial gains to close down as caution set on weak consumer sentiment figures. Australian consumer confidence fell in May on pessimism over the government's federal budget announcement.

The news dealt a 3 percent blow to shares of the nation's largest department store Myer, which were already hurt by the firm's gloomy outlook for retail trading.

Seven West Media tanked 8 percent on news that U.S. private equity firm Kohlberg Kravis Roberts (KKR) will be selling its remaining stake in the Australian media firm.

Kospi Up 0.6%

Seoul's benchmark index snapped a three-day losing streak as blue-chip exporters led gains with automakers in the spotlight.

Hyundai Motor rose 2 percent and Kia Motors rallied 4.6 percent after South Korean exports for April came in flat from a year earlier, soothing investor fears that the impact from the yen's dramatic slide on local exporters' profits was less severe than expected.

Samsung Electronics jumped over 1 percent after announcing it will acquire a 10 percent stake in local smartphone maker Pantech.

By's Nyshka Chandran. Follow her on Twitter @NyshkaCNBC