BEIJING - China National Offshore Oil Corp (CNOOC) said Monday it has received a report from ConocoPhillips, the operator of the Penglai 19-3 oilfield, that a small amount of oil had leaked into the sea during oil transferring operations on Sunday.

While transferring oil, the safety device of the hose connection valves automatically disconnected due to abnormalities, leading to leakages estimated at a maximum of 0.6 tons, according to CNOOC.

The oil source had been immediately cut off when the automatic isolation valves started to work, CNOOC said.

After the incident, ConocoPhillips had carried out clean-up work and no more oil slick had been monitored as of 3 pm Monday.

The Penglai 19-3 oilfield is one of China's largest offshore oilfields, with a daily production of about 160,000 barrels. ConocoPhillips China operates the Penglai 19-3 oilfield, in which CNOOC, China's largest offshore oil producer, holds a 51-percent stake while ConocoPhillips holds 49 percent.