U.S. private equity firm Apollo Management unveiled a $6 billion bid proposal for Huntsman Corp on Wednesday, 8 percent higher than a deal accepted by the U.S. chemical company last week.

Huntsman, whose products are used in paints, footwear and cleaning products, said the new proposal was from Apollo's Hexion Specialty Chemicals Inc and was worth $27.25 per share.

On June 26, privately owned Dutch chemicals maker Basell said it had agreed to buy Huntsman for $5.6 billion excluding debt, or $25.25 per share, in a bid to strengthen its position as a global chemicals group.

Huntsman shares, at $18.90 before news of the Basell deal, closed at $24.40 on Tuesday. U.S. markets are closed for a holiday on Wednesday.

Basell, which would not comment on Apollo's offer, is entitled to a $200 million break-up fee from Huntsman if it walks away from the deal, Huntsman said, adding Hexion had offered to fund $100 million of this payment and has funding in place for its proposed acquisition.

HSBC analyst Hassan Ahmed said at the time of the Basell deal that it represented great value for Huntsman shareholders as it valued the firm at about 8.6 times earnings before interest, taxes, depreciation and amortization (EBITDA).

Basell, owned by privately-held U.S. industrial group Access Industries, is a leading producer of polypropylene used in textiles, reusable containers and laboratory equipment. Its bid for Huntsman is worth $9.6 billion including debt.

Access itself is run by Len Blavatnik, a U.S. businessman of Russian origin estimated by Forbes to be the world's 40th richest person in 2006 with a $7.0 billion fortune.

Entities controlled by private-equity fund MatlinPatterson and the Huntsman family, who collectively own 57 percent of Huntsman's common stock, have agreed to the deal with Basell.

The cost of insuring Basell's debt fell on Wednesday, with five-year CDS on Basell down 25 basis points to 385 b.p.

Ratings agencies Moody's Investors Service, Standard & Poor's and Fitch Ratings have said they may cut Basell's ratings if its debt-funded acquisition of Huntsman goes through.

Hexion's proposal includes a clause in which it will pay Huntsman $325 million if the transaction does not close due to the failure to obtain regulatory clearance or requisite financing.

(Additional reporting by Richard Barley in London and Foo Yun Chee in Amsterdam)