Weyerhaeuser

(WY) - Get Report

said Monday it is extending its hostile, $5.4 billion cash tender offer for shares of

Willamette Industries

(WLL) - Get Report

to May 18.

The $48-a-share offer was due to expire on March 30.

Willamette, a lumber manufacturer based in Portland, has shot down the bid

numerous times in past months, calling it a "

low-ball offer" in a February letter to Weyerhaeuser, a paper products company.

In December, Willamette's board

unanimously rejected Weyerhaeuser's takeover bid.

As of noon Monday, Willamette shareholders had tendered roughly 31.4 million shares pursuant to Weyerhaeuser's offer. Willamette has 109.42 million shares outstanding.

Willamette's shares closed down $1.05, or 2.2%, at $47 on the

New York Stock Exchange

. Weyerhaeuser was off $3.70, or 6.5%, at $53.35.