Chinese state-owned energy giant CNOOC Tuesday said its first-half net profit fell 19 percent year-on-year amid rising costs and a drop in production after the closure of a major oil field.

Net profit for the six months to June fell to 31.87 billion yuan ($5.01 billion), down from 39.34 billion yuan a year ago, China's largest offshore oil and gas producer said in a filing to the Hong Kong stock exchange.

CNOOC, the Hong Kong-listed unit of state-owned China National Offshore Oil Corporation, said its revenue also fell 5 percent to 118.27 billion yuan during the period.

The energy giant said its net output fell 4.6 percent due to suspension of production at the Penglai 19-3 oilfield in China's Bohai Bay, which it co-owns with US oil giant ConocoPhillips, after an oil leak.

It warned of a challenging second-half, with chief executive Li Fanrong saying the external environment will remain "critical".

"For the second half of the year, the world economy will likely continue to bear the downward pressure and international oil prices are expected to become increasingly volatile," Li said in a statement.

He however said CNOOC, which made 10 discoveries in the first-half of the year, was confident that it will reach its annual production target of 330-340 million barrels of oil equivalent.

CNOOC announced a $15.1 billion bid last month to buy Canadian oil firm Nexen -- which would be China's largest foreign investment and its largest energy deal if approved by regulators, according to data firm Dealogic.

However the proposed takeover was hit by insider trading claims just days after the bid was announced.

US regulators claimed Hong Kong-based Well Advantage Limited, controlled by businessman Zhang Zhirong and other unidentified traders, reaped more than $13 million after they stockpiled shares of Nexen stock "based on confidential information about the deal".

China is the biggest energy consumer in the world, the second-biggest consumer of oil and has been snapping up resource assets across the globe in order to fuel break-neck growth.