Asian stocks slipped on Wednesday, taking cues from an overnight decline on Wall Street and capped by caution ahead of a policy statement from the US Federal Reserve due later in the day.

The Australian dollar, already on the back foot following a slide in crude oil, dropped to a 3-week low after soft Australian inflation data paved the way for a further rate cut.

"You have to say data like this coupled with what the banks have done recently with the tightening up in conditions must increase the risk of a move before year end," said Su-Lin Ong, a senior economist at RBC Capital Markets. She was referring to Australia's major banks raising their variable mortgage rates to offset more stringent regulatory capital requirements.

MSCI's broadest index of Asia-Pacific shares outside Japan declined by 0.6 per cent. Shanghai stocks dipped 0.1 per cent, Hong Kong's Hang Seng fell 0.3 per cent and South Korea's Kospi dropped 0.3 per cent.

Tokyo's Nikkei bucked the trend and rose 0.6 per cent on bargain hunting following the previous day's fall.

On Wall Street, the Dow fell 0.2 per cent and the S&P 500 retreated 0.3 per cent.

Closely watched earnings from Apple Inc out late on Tuesday painted a rosy picture for the new iPhones, but a quarterly slowdown of overall sales in China cast doubt on the robustness of Apple's legendary profitability.

Apple shares initially rose after hours as it beat sales and profit forecasts, but they gave up those gains later as concerns crept in.

In currencies, the dollar treaded water against the yen and euro before the Fed's policy decision is known.

"No one expects the Federal Reserve to hike on Wednesday and we would not be surprised if they refrained from providing any clear signal about their intention to raise interest rates before the end of the year," wrote Kathy Lien, director of FX strategy for BK Asset Management.

Focus fell on the Fed's stance after the European Central Bank opened the door for more easing and China cut rates and reserve requirements.

"While it can be argued that stimulus abroad is good for US markets and makes it easier for the Fed to raise interest rates in December, the reasons why these central banks are easing and the consequences for the dollar could also deter them from tightening," said BK Asset Management's Lien.

The dollar was steady at 120.44 yen while the euro inched down 0.2 per cent to $1.1029.

Commodity currencies like the Canadian dollar were hit by a slide in crude oil prices. The dollar was steady at C$1.3274 after surging 0.9 per cent overnight.

The Australian dollar struggled near a 3-week low of $0.7118 , having lost about 0.7 per cent on the day.

US crude oil nudged up a touch to $43.29 a barrel after sliding 1.7 per cent overnight to a two-month low, ahead of official inventories data due later in the session that are expected to confirm the persistent supply glut dogging the market.

Brent crude was little changed at $46.84 a barrel following a 1.5 per cent decline overnight to a mid-September low. – Reuters