German drug and crop chemical group Bayer AG on Thursday announced details of a sweetened US$64 billion bid for Monsanto Co as it tries to put the U.S. seed company under pressure to engage further.

The move shows the two companies have made little progress in their negotiations since Monsanto rejected Bayer's previous offer in May while saying it was open to "continued and constructive conversations."

Monsanto Chief Executive Officer Hugh Grant said last month that the 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.

Bayer said on Thursday that it had raised its offer to US$125 a share from US$122 in cash and offered Monsanto a US$1.5 billion reverse antitrust breakup fee, "reaffirming its confidence in a successful closing."

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

Monsanto said its board would review Bayer's latest proposal, in consultation with its financial and legal advisors.

Shares of Monsanto were up 2.9 per cent at US$104.03 in afternoon trading in New York, while Bayer closed up 0.5 per cent at 93.44 euros (US$103.88) in Frankfurt.

Monsanto may respond to Bayer's latest offer as early as this week, according to people familiar with the matter. They asked not to be identified discussing confidential deliberations.

While Monsanto has shared more information with Bayer since May, it has yet to provide it with a confidentiality agreement that would allow the German company to go over its books, the sources said.

Bayer's relatively modest price increase in its offer also reflects the view that Monsanto's recent poor earnings have weighed on its valuation.

Monsanto said last month that its net income tumbled more than 37 per cent to US$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.