The Hang Seng Index fell 194 points or 0.83 percent to open at 23,335 points on Monday.

Mainland stocks also opened lower, with the benchmark Shanghai Composite Index down 0.31 percent to open at 3,349 points.

The Shenzhen Component Index opened 0.55 percent lower at 9,950 points.

Meanwhile, oil prices jumped on Monday to their highest level since January as the United States' weekend move to join Israel in attacking Iran's nuclear facilities stoked supply worries.

Brent crude futures was up US$1.92 or 2.49 percent at US$78.93 a barrel as of 0117 GMT.

US West Texas Intermediate crude advanced US$1.89 or 2.56 percent to US$75.73.

Iran is OPEC's third-largest crude producer. (Agencies)