Monsanto Co (MON.N) has turned down a sweetened $64 billion acquisition offer from Bayer AG (BAYGn.DE), but said it was open to further talks with the German healthcare and chemicals group as well as other parties.

The widely expected rejection puts pressure on Bayer to sweeten its offer once again, at least enough to get access to Monsanto's books. The two companies have been in negotiations about a potential confidentiality agreement, Reuters reported on Monday.

Monsanto said on Tuesday its board unanimously viewed Bayer's latest bid as "financially inadequate and insufficient to ensure deal certainty."

"Monsanto remains open to continued and constructive conversations with Bayer and other parties to assess whether a transaction that the board believes is in the best interest of Monsanto share owners can be realized," the company said.

Bayer said it was disappointed with Monsanto's decision to reject its latest offer but was looking forward to continued dialogue with Monsanto under an appropriate confidentiality agreement that would allow access to additional information.

Access to confidential information has been a major sticking point in Bayer's negotiations with Monsanto ever since the German company offered to acquire Monsanto in May.

Bayer disclosed Thursday that it had increased its bid by $3 per share. Its latest $125-per-share offer is the largest all-cash bid on record. Bayer also offered a $1.5 billion reverse antitrust breakup fee, equivalent to about 2.3 percent of the deal's value.

By comparison, the breakup fee ChemChina offered to acquire Swiss peer Syngenta (SYNN.S) should the deal not go through is $3 billion, representing 7 percent of the $43 billion transaction value.

Monsanto would like Bayer to sweeten its offer further, including increasing the breakup fee, before providing Bayer access to financial information, according to people familiar with the matter who asked not to be identified discussing private discussions.

Monsanto's shares were up 0.35% to $106.81, while Bayer shares were down 1.2% at 91.87 euros.

Bayer argued last week that it had comprehensively addressed Monsanto's questions about financing and regulatory matters and said it was prepared to make certain commitments to regulators, if required, to complete a deal.

Will Bayer Overpay? 

Henderson Global Investors (HGGH.L), an investor in Bayer, has called for a vote on the proposed takeover of Monsanto, which it said threatened the long-term strength of the German company. Some other Bayer investors have also expressed concerns the company may overpay to secure a deal.

Monsanto Chief Executive Hugh Grant said last month that his company was in talks with Bayer and other companies in its sector about "alternative strategic options." He did not name the other companies, but Reuters has previously reported that Monsanto had discussed a business combination with BASF SE (BAYGn.DE).

The seeds and agrochemicals industry has been jolted by several large deals in the past year as low crop prices and belt-tightening by farmers pressured earnings.

Syngenta AG, which Monsanto tried to buy last year, agreed in February to be acquired by ChemChina for $43 billion. Dow Chemical Co (DOW.N) and DuPont (DD.N) struck a $130 billion mega-merger late last year.

Monsanto said last month net income tumbled more than 37 percent to $717 million in the quarter ended on May 31. It cited a global glut of generic glyphosate, the active ingredient in its Roundup herbicide, and delays in securing European Union import approval for its next-generation biotech soybeans.