AMCOL International Corp. has approved an amended takeover offer from Imerys worth $45.25 per share, narrowly topping a rival bid and again raising the stakes in a bidding war over the U.S. bentonite producer.

In the latest twist, AMCOL agreed to approve an amended merger agreement with Imerys, which is now paying $2.50 per share more than what was approved late last month.

The French company has now raised the offer price for AMCOL twice in less than a month, as it faces pressure from a competing proposal by Minerals Technologies Inc. On Monday, Minerals Technologies boosted its own bid for AMCOL to $45 a share.

A representative at Minerals Technologies wasn't immediately able to comment on Imerys' latest purchase price.

AMCOL, which posted sales of more than $1 billion in 2013, is the world's largest producer of bentonite, a mineral used in many industries such as car manufacturing and heavy machinery.

AMCOL's shares weren't active in after-hours trading. The stock jumped last month when a takeover offer by Imerys was unveiled, and is up 36% in 2014 as of Tuesday's close.

Write to John Kell at john.kell@wsj.com

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