* MSCI Asia ex-Japan pressured, Nikkei opens down 1.6 pct
* China official services PMI, US national ISM eyed
* Dollar steady vs yen, dollar index softer
Asian shares began the new month with a cautious tone on Monday as uncertainty over how much longer the current U.S. stimulus would continue prompted investors to book profits from recent highs and pulled global equities lower.
Speculation over whether and when the U.S. Federal Reserve would start scaling back its current massive bond-buying programme emerged following a string of positive U.S. data and was the catalyst for corrections across markets which had drawn strong support from the Fed's largesse.
Investors will be cautious ahead of more data this week from the United States as well as from China, both of which would offer clues to growth and demand prospects in the world's largest economies.
"The Federal Reserve's willingness to alter the pace of purchases sooner than markets had previously expected has meant that markets remain attuned to the incoming data flow in the U.S.," Barclays Capital said in a research note.
"This week brings the potential for more market-moving data in the U.S.," it said, referring to the May ISM manufacturing index due later in the session and the more important monthly nonfarm payrolls data due on Friday. The Fed has said it would keep up the stimulus campaign until the employment situation improved.
MSCI's broadest index of Asia-Pacific shares outside Japan remained pressured, after falling to its lowest in nearly seven weeks on Friday and ending May down 4.7 percent for its worst monthly performance in a year.
Australian shares were down 0.2 percent while South Korean shares opened down 0.4 percent.
Even Japan's Nikkei stock average, which has outperformed the Asian bourses, eased, opening down 1.6 percent and hit a six-week low early on Monday.
"Investors are mostly likely to stay on the sidelines because we have a lot of important data this week from the States," said Takashi Hiroki, chief strategist at Monex Inc.
The Standard & Poor's 500 Index posted consecutive weekly losses for the first time since November as investors took some money off the table after the index rallied 14.3 percent, the best first five months of the year since 1997, and ended May up for the seventh straight month of gains - its longest streak of monthly gains since 2009.
Reflecting heightening investor nervousness, the CBOE Volatility index, which measures expected volatility in the Standard & Poor's 500 index over the next 30 days, hit a six-week high on Friday.
Data on Friday showed U.S. Chicago Purchasing Managers Index rose far more than expected in May, released ahead of the national manufacturing data due later this session.
The strong business activity data fanned worries about the Fed slowing its bond purchases later this year and sent U.S. Treasury prices lower on Friday, capping the worst month for the market in nearly 2-1/2 years.
Later in the Asia session, China is set to release the official services PMI and the final HSBC survey that focuses on smaller private sector firms. Over the weekend, China said its official PMI rose to 50.8 in May from 50.6 in April, beating market expectations and raising optimism that the world's second-largest economy may be stabilising.
"While the data may ease some concerns of rapid deteriorating of the Chinese economy, the impact may be short-lived," ANZ said in a commentary, noting that this single indicator did not change its view on China's softening economic condition.
"Structural reforms are needed in order to help sustain the growth prospect ... inconsistent data will continue to complicate China's economic policy making and potentially impair the judgment of policymakers," ANZ added.
The dollar was steady around 100.40 against the yen, having hit a three-week low of 100.22 on Friday. The dollar index, measured against a basket of six key currencies, was down 0.11 percent after touching a three-week low on Thursday.
U.S. crude futures fell 0.6 percent to $91.43 a barrel.
Amid global equity market corrections spurred by concerns over the Fed's stimulus outlook, EPFR Global said on Friday the equity funds it tracks recorded collective outflows of $2.79 billion during the week ending May 29 as retail redemptions hit a year-to-date high while bond funds attracted $1.37 million, their second lowest total of 2013.