The deadline for the takeover of Nexen Inc by China's largest offshore oil producer has been extended by 30 days, it was announced on Monday.

The China National Offshore Oil Corporation (CNOOC) Ltd has received approval by the Canadian government to buy Calgary-based oil and gas producer Nexen Inc for $15.1 billion.

A statement said the closing date for the takeover would be postponed from Jan 31 to March 2, 2013. According to their previous agreement, the closing date can only be extended by maximum of 75 business days beyond the original deadline.

The deadline is being extended because the two companies are still waiting for approval from US regulators which is essential for completing the deal, as Nexen has operations in the Gulf of Mexico, the statement said.

Authorities in the United Kingdom, the European Union and China have approved the deal, Nexen said in a statement on Sunday. Nexen also said it will delay the release of its 2012 fourth quarter earnings and its annual financial report. The takeover will be China's biggest overseas acquisition.