Asian markets kept their nerve on Wednesday counting on the Federal Reserve to launch only a modest scaling back of stimulus later in the day, though all assets were vulnerable to any hint of hawkishness from the world's most powerful central bank.

Expectations are that the Federal Open Market Committee (FOMC) will be measured with any cuts to its $85 billion in monthly asset buying, while also seeking to reassure investors that the day of an actual policy tightening is still distant.

That kept the dollar pinned near a four-week trough against a basket of major currencies. The euro idled at $1.3350 , near the week's peak of $1.3385.

On the yen, the greenback inched ahead to 99.24 and near the middle of its recent 98.45-100.62 range.

Caution ruled in most stock markets, with MSCI's broadest index of Asia-Pacific shares outside Japan off a slight 0.3 per cent. Shanghai shares eased 0.2 per cent while Australia lost 0.4 per cent.

Japan's Nikkei was the main standout with a jump of 1.8 per cent, reaching its highest since late July.

Overnight, the Dow Jones industrial average eked out a 0.2 per cent gain, while the S&P 500 added 0.42 per cent. Europe's FTSEurofirst 300 index fell 0.46 per cent.

For the Fed, consensus has congealed around a reduction of $10-$15 billion a month with all purchases ending by the middle of next year. Yet even that cautious timetable would be contingent on the economy performing as well as expected.

With such an outcome largely priced in, it could lead Treasuries and the dollar to rally modestly. A slower tapering would tend to benefit bonds and stocks but hurt the dollar.

The bigger reaction would likely come if the Fed pulled back more aggressively, as that would lead market to price in an earlier start to rate rises as well.

That would be especially painful for emerging market countries that rely on foreign capital to fund current account deficits, with India and Indonesia among the most vulnerable.

The tension was evident in Jakarta where both shares and the rupiah came under pressure.

A LOT TO CONSIDER

Still, dealers warned against a hasty reaction as there were so many moving parts in play.

As well as the tapering, the Fed may choose to alter its threshold for tightening, perhaps by lowering the trigger level on unemployment from the current 6.5 per cent.

It will also publish its first economic forecasts for 2016 and the stronger the picture the harder it will be to convince markets that any future rise in interest rates will only be slow and measured.

Indeed, the Fed has already had trouble convincing the market that it intends to keep rates near zero out to 2015 no matter how much the economy improves.

The decision and economic projections are out at 1800 GMT while Fed Chairman Ben Bernanke starts his press conference half an hour later. Often markets can react violently to the former, then completely reverse course depending on what Bernanke says.

"We expect Bernanke's press conference to be dovish. The Fed will want to temper market expectations that tapering will be rapid or that FOMC participants have brought forward their expectations for the first increase in rates," says Joseph Capurso, currency strategist at Commonwealth Bank of Australia.

"While the dollar may soften after the FOMC meeting, our medium term view of a stronger dollar is unchanged," he added, citing higher US yields and the diverging outlook for rates between the United States and other rich nations.

While yields on 10-year Treasury notes were a tick lower at 2.84 per cent on Wednesday, that is up from just 1.62 per cent back in May before the Fed first raised the spectre of tapering.

For commodities, the more dovish the outcome from the Fed the more supportive for prices. Copper futures were a shade firmer at $7,088.00 on Wednesday, though still down 11 per cent for the year so far.

Spot gold shed 1 per cent to $1,297.20 an ounce as dealers took a "just in case" attitude to the Fed.

Brent crude for delivery in November fell 19 cents to $108.00 a barrel, but US crude for October delivery added 24 cents to $105.66 a barrel. - Reuters