SINGAPORE - Shares of upstream energy exploration firm Rex International Holding surged after the company announced a successful exploration in Oman, though the Singapore market was on course for a fourth straight day of declines.

Singapore-listed Rex International's shares were headed for their biggest gain since September, soaring as much as 10.8 per cent to $0.67, after the company said its indirectly-owned subsidiary, Masirah Oil, discovered hydrocarbons in its second exploration in the east of Oman.

The stock was heavily traded at almost three times its average 30-day full-day volume as of midday, with more than 20.1 million shares changing hands.

Singapore shares tumbled to their lowest since November 2012 after disappointing United States manufacturing data dealt another blow to a market already worried over a possible flight of capital from emerging markets after the Federal Reserve's decision to continue tapering its bond-buying program.

The benchmark Straits Times Index was down 0.7 per cent at 2,970.8 by midday, while the MSCI's broadest index of Asia-Pacific shares outside Japan fell 1.4 per cent.

Shares of SIA Engineering Co, the index's worst performer, were on course for their biggest daily decline in six months, dropping 3.3 per cent to an intra-day low of $4.75. The stock reached its lowest in more than 4 months after the company on Monday reported a 9.7 per cent drop in its third-quarter net profit.

Brokerage Maybank Kim Eng maintained its "buy" rating on SIA Engineering with a target price of $6.34, despite the disappointing earnings.

"It is the best proxy to the unprecedented level of expansion at Changi Airport," Maybank Kim Eng said in a report.

"Its network of associates and joint ventures would also continue to provide a solid stream of earnings and return cash dividends to shareholders."