DALLAS — Apollo Management LP raised its takeover bid for rival chemical company Huntsman Corp. to about $6.5 billion on Monday, sweetening an offer that was already higher than a competitor's proposal.

Apollo raised the bid by its Hexion Specialty Chemicals Inc. unit by 2.7 percent even though the Huntsman board had judged its earlier offer superior to one that Salt Lake-based Huntsman had already accepted from the Dutch company Basell.

It wasn't clear whether Apollo raised its bid to impress the Huntsman board — which was still recommending shareholders accept a sale to Basell — or if it was trying to pre-empt a counteroffer from Basell.

A spokesman for Apollo declined to comment. A spokesman for Basell wouldn't say whether the Dutch firm had been preparing a new offer — or would respond to the latest move by Apollo. Basell has until the close of business today to get back in the bidding.

"We are considering our options, and we'll decide over the next few days what our plan is," said the Basell spokesman, Stan Neve.

The bidding for Huntsman is among the latest in a wave of deals in the chemicals industry, as companies try to bulk up.

Also on Monday, London-based private equity firm CVC Capital Partners bid $2 billion for Dutch chemicals distributor Univar NV. The offer, a 37 percent premium over Univar's closing stock price on Friday, was supported by the Univar board.

In March, Univar agreed to pay $600 million for privately held Chemcentral Corp. of Bedford Park, Ill., to bolster U.S. sales. About two-thirds of Univar's business is in the United States, with the rest in Europe. The company had sales of $6.6 billion last year.

Basell, a Dutch holding company controlled by U.S. industrialist Len Blavatnik's Access Industries Inc., had the early inside track on bidding for Huntsman.

Basell offered $25.25 per share. But Apollo topped that by offering $27.25 per share.

On Monday, Apollo's Hexion raised its offer to $28 per share, or more than $6.5 billion after factoring in about 233 million diluted shares of Huntsman, said an Apollo spokesman, Jonathan Gasthalter. Apollo would also assume about $4 billion in Huntsman debt.

Hexion said Friday that Huntsman had determined that its bid was superior to the Basell offer. Hexion said Huntsman told Basell it would continue to recommend the Dutch firm's bid to shareholders for three business days, or the close of business on Tuesday, after which it could oppose the deal — essentially inviting Basell to sweeten its offer.