Endeavour International Corp said its unit has agreed to buy ConocoPhillips' interest in three producing U.K. oil fields for $330 million (210 million pounds), bulking up the U.S. oil and gas exploration company's North Sea assets.

Shares of Endeavour shot up as much as 13 percent to a three-week high of $7.23 in early trading on the New York Stock Exchange.

Current net production from the assets is about 10,000 barrels of oil equivalent per day. Endeavour estimates the assets' proved and probable reserves as of June end are in excess of 33 million barrels of oil equivalent.

Endeavour, which expects to close the deal in the first half of 2012, said the acquisition will add ownership interests in the Alba, MacCulloch and Nicol fields.

(Reporting by Swetha Gopinath in Bangalore; Editing by Maju Samuel)