The Hang Seng Index lost over 100 points, or 0.42 percent, to open at 23,791 on Monday.

Mainland stocks also opened lower, with the benchmark Shanghai Composite Index down 0.23 percent to open at 3,369.

The Shenzhen Component Index opened 0.25 percent lower at 10,096.

Investors were on edge as financial markets reopened, with crude oil prices initially up near 4 percent as markets were gripped by the escalating threat of a sweeping conflict in the Middle East.

Meanwhile, data released on Monday showed China's industrial output rose 5.8 percent year on year in May, slowing from the 6.1 percent pace in April.

Retail sales growth unexpectedly picked up, rising 6.4 percent in May, compared to 5.1 percent growth in April.

It was the quickest growth since December 2023.

Elsewhere in the region, Japan's Nikkei firmed 0.8 percent and South Korean stocks added 0.5 percent in morning trading. (Xinhua)