Updated with new information.

South San Francisco, Calif. (

TheStreet

) --

Onyx Pharmaceuticals

(ONXX)

confirmed receipt of a takeover offer from

Amgen

(AMGN) - Get Report

valued at $120 per share but said the bid was too low. Onyx will now seek competitive bids from other interested acquirers, the company said Sunday.

The

Financial Post

first broke the story Friday night of Amgen's interest in Onyx, citing an unsolicited bid letter.

"I feel a timely combination of our organizations would be very complementary," said the letter written by Amgen to Onyx two weeks ago, according to the

Financial Post story

. The letter goes on to propose an Amgen acquisition of Onyx for $120 per share, or $10 billion in total.

Onyx shares jumped 19% to $107.50 in Friday after-market trading on news of the Amgen bid.

On Sunday, Onyx said the Amgen offer "significantly undervalued" the company and was not in the best interests of shareholders.

"Based upon expressions of interest received from other third parties and the recent proposal from Amgen, the Onyx Board has authorized its financial advisor to contact potential acquirers who may have an interest in the Company in a transaction that is in the best interest of Onyx shareholders," the company stated.

Amgen shares were unchanged at $96.88 in after-hours trading.

-- Reported by Adam Feuerstein in Boston.

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