WELLINGTON/MUMBAI (REUTERS, BLOOMBERG) - Asia central banks signalled major concerns on Wednesday (Aug 7) about the outlook for economic growth, stunning markets with a series of surprising interest rate cuts and pointing to policymakers' dwindling ammunition to fight off a downturn.

New Zealand's central bank cut its official cash rate 50 basis points to a record low of 1 per cent. That caught off guard investors who expected a 25 basis point cut, prompting a 2 per cent fall in the country's currency.

The Bank of Thailand followed suit, cutting its one-day repurchase rate by 25 basis points to 1.5 per cent. All but one of 15 analysts in a Reuters poll had predicted no rate change.

Citing growth concerns as "the highest priority", the Indian central bank cuts its repo rate a bigger-than-expected 35 basis points to 5.4 per cent.

Wall Street stocks finished little changed on Wednesday after a wild ride as a plunge in treasury bond yields early in the day underscored worries about a weakening global economy.

The Dow Jones Industrial Average rallied back from a 589-point drop in a wild session to end down 22.45 points, or 0.09 per cent, to 26,007.07. The S&P 500 gained 2.21 points, or 0.08 per cent, to 2,883.98 and the Nasdaq Composite added 29.56 points, or 0.38 per cent, to 7,862.83.

Earlier in Washington, US President Donald Trump stepped up his relentless assault on the Federal Reserve in a series of Wednesday morning tweets that renewed his demand for "bigger and faster" interest-rate cuts.

"Incompetence is a terrible thing to watch, especially when things could be taken care of sooo easily. We will WIN anyway," he said. "It would be much easier if the Fed understood, which they don't, that we are competing against other countries, all of whom want to do well at our expense!"

"They must Cut Rates bigger and faster, and stop their ridiculous quantitative tightening NOW. Yield curve is at too wide a margin, and no inflation!" Trump said. The Fed last week ceased its gradual shrinking of the balance sheet, which the president calls quantitative tightening.

He also called attention to three central banks that cut rates earlier on Wednesday, though he didn't name them in the tweet, and said, "Our problem is not China - We are stronger than ever" before returning to his familiar complaint against the US central bank.

"Our problem is a Federal Reserve that is too proud to admit their mistake of acting too fast and tightening too much (and that I was right!)"

China's central bank meanwhile on Wednesday denied, and dismissed as untrue, rumours circulated online about its decision to cut the benchmark deposit and lending interest rates from Aug 10, saying it had asked police to investigate.

The People's Bank of China made the statement on its official social media account, but gave no other details.

China let the yuan breach the key 7-per-dollar level on Monday for the first time in more than a decade, a sign Beijing might be willing to tolerate more currency weakness that could further inflame a trade conflict with the United States.

The Asia rate cut moves came after trade tensions and slowing global growth prompted the Fed to cut rates for the first time in a decade and the European Central Bank to signal policy easing. Australia's central bank eased policy in June and July.

"Almost every tool in the box has been used to engineer a soft landing in the current economic cycle downturn," analysts at OCBC in Singapore said in a market note, arguing that limited stimulus options, given that rates are generally very low, were problematic.

"We may stave off this recession with rate cuts - but when the accumulation of stresses comes due in the next downturn, the lack of monetary policy space is likely to result in a harder landing for the global economy."

Market volatility this week as the US-China trade war ramped up has added to concerns for central banks. In South Korea, policymakers said they stood ready to cut rates if needed to deal with the impact of anxious markets.

Pointing to similar concerns over the global economic outlook, Bank of Japan (BOJ) policymakers called for the central bank to talk about increased stimulus, according to a summary of views from the July policy review.

With interest rates hovering around zero, the BOJ is left with little ammunition to fight a downturn. Prolonged ultra-low rates have also heightened the cost of additional easing by hurting margins for banks.

After cutting rates, the governor of New Zealand's central bank, Adrian Orr, pointed to the risk that the central bank will have to do much more to counter the slowdown in economic growth.

"It is easily within the realms of possibility that we might have to use negative interest rates," Orr told a news conference after Reserve Bank of New Zealand's policy meeting.

Minutes of the meeting highlighted how alarmed policymakers had become by the escalation in the Sino-US trade dispute, fearing its deadening impact on investment and growth.

"This was a stunning decision," said Westpac's NZ chief economist Dominick Stephens, noting New Zealand had cut rates by 50 basis points or more on only three other occasions.

OFFENCE BEATS DEFENCE

Policymakers everywhere have been forced to consider more stimulus as fears grow over the broadening fallout of the US-China trade dispute on the global economy.

This week, investors feared that the trade war was turning into a currency war as Beijing allowed its currency to slip to the weaker side of 7 per dollar for the first time in more than a decade and Washington labelled the country a currency manipulator.

Markets are wagering that the ECB and BOJ will have to ease policy, otherwise their currencies could rise to uncompetitive levels.

"Offence is the best form of defence," Josh Williamson, an economist at Citi, said of the New Zealand central bank's rate cut.

"Committee members were more concerned about global growth headwinds and the potential impact on the New Zealand economy via the trade channel."