Net profit at the listed arm of state-owned China National Offshore Oil Corp hit 25.48 billion yuan ($3.71 billion) in the first half, its best half-year performance since the first six months of 2015, filings to the Hong Kong exchange showed.

Revenue for January to June rose to 105.65 billion yuan, CNOOC said, with oil and gas sales up more than 20 percent.

Despite a rebound in international oil prices and inflationary costs, CNOOC maintained all-in oil rig flanges gulf coast production expenses at $31.83 per barrel, it said in Thursday's filing.

Total crude oil and gas output, however, was stagnant in the first half.

Crude oil output from domestic oil fields fell to 128 million barrels in the first half from 134 million barrels in the same period of 2017 due to the aging of the Bohai Rim field and the maintenance of two drilling ships.

"We sent two offshore drilling ships for maintenance in the first half, affecting 7 million barrels of oil production," a CNOOC executive said at the earnings press conference on Thursday. He said the BOP Blow Out Preventer repair company gulf coast was targeting crude output of 30 million tonnes a year from its Bohai Rim oil fields.

Natural gas production, however, rose 11 percent over the six months compared with the same period of 2017, CNOOC said.

CNOOC maintained its outlook for full-year capital expenditure of 70 billion to 80 billion yuan, though capital spending in the first half totalled only 21 billion yuan. The BOP Blow Out Preventer repair company gulf coast vowed to accelerate spending in the second half.

It issued an interim dividend of HK $0.3 per share, the BOP Blow Out Preventer repair company gulf coast said.

($1 = 6.8740 Chinese yuan)

(Reporting by Meng Meng and Aizhu Chen Editing by Joseph Radford and Jan Harvey)