By Chao Deng 

Asia shares dropped Monday amid jitters about upcoming Chinese manufacturing data and the continued declines in commodity prices.

The Shanghai Composite opened 1.3% lower at 3614.99, while the smaller Shenzhen Composite traded down 1.5% at 2079.13. The small-cap ChiNext board slipped 2.5% to 2475.33.

The Nikkei Stock Average was down 0.8%, Australia's S&P ASX 200 lost 0.4% and South Korea's Kospi was down 1%.

In China, investors in the region await a final reading of the Caixin manufacturing PMI for July later this morning.

The official manufacturing PMI, released on Saturday, slipped, pointing to further sluggishness in the key manufacturing sector of the world's second-largest economy. The reading fell to 50.0 in July from 50.2 in June, according to the National Bureau of Statistics.

The reading was disappointing given that "the official PMI bottomed in January and had been grinding higher," Tim Condon, a strategist at ING, wrote in a report Monday. Still he added, "We believe the stock market panic in early July chilled economic activity, which is what the manufacturing PMIs picked up. The authorities quelled the panic, which we think will make the PMI plunges transitory."

In July, the Shanghai Composite suffered its worst month in nearly six years, revealing a receding confidence in Beijing's ability to stem a selloff that began in mid-June. The Shanghai Composite Index lost 14% in July, including 1.1% on Friday.

Losses in the commodities markets Monday came after big oil companies pulled the Dow Jones Industrial Average lower Friday, on disappointing earnings reports and signs of increased U.S. oil drilling.

Brent crude, already at multi-month lows, was last down 0.7% in Asia trade at $51.84 a barrel. Gold was last down $2.30 at $1,092.70 an ounce.

"The biggest question for the month is whether we give back the gains from July," IG market strategist Evan Lucas said of the Australia's market. This week sees Australia's earnings season begin in earnest, with companies including Suncorp Group Ltd., Virgin Australian Holdings Ltd., and Rio Tinto Ltd. all set to turn in numbers. The ASX 200 climbed 4.4% last month, the first monthly rise since February.

Shares in Japan fell even as the yen weakened. The currency last traded at 124.08 from 123.92 late Friday in Asia.

A softer yen, in addition to the benefits of growing North American demand and higher spending by foreign tourists, however, has helped Japanese corporates, which have so far reported robust profit growth in the April-June quarter. Of the 596 nonfinancial companies closing their books in March that reported earnings by the end of July, 70% saw pretax profit increases on the year, The yen average slightly more than 121 to the dollar in April-June, nearly 20 yen weaker than a year earlier.

Hiroyuki Fukunaga, CEO at Investrust, said Japan stocks are likely to continue trading in lackluster fashion before anxiously awaited U.S. jobs data is delivered on Friday.

"The U.S. Federal Reserve bank has been very reticent to declare September as a definite 'go' time for raising interest rates; it remains heavily data-reliant," he said . "Thus Friday's jobs data is seen as critical to formulating the decision on timing. Investors are expected to remain largely in a holding pattern this week as a result."

Futures for the Hang Seng Index were down 0.4% at 24533.14.

The latest measures by Chinese regulators to stem selling include an investigation into suspicious activity of trading on the mainland. China's securities regulator said Friday it has launched a probe into automated trading and has restricted 24 stock accounts suspected of influencing stock prices. The government didn't name any of the parties behind the restricted stock accounts, but U.S.-based hedge fund Citadel LLC said trading in one of the accounts it manages in China has been suspended.

Write to Chao Deng at Chao.Deng@wsj.com