The Monsanto takeover by Bayer is the result of several months of talks and is thought to value Monsanto at more than $66bn (£50bn).
NEW DELHI: According to German media, Bayer has increased its offer to $129 per share while the Reuters news agency says the deal will be announced Wednesday.
The takeover would create the world's biggest seeds and pesticides company.
On 5 September, Bayer made its latest increased offer of $127.50 per share. Monsanto rejected this, but said it was engaged in "constructive negotiations" with Bayer.
The latest $66bn offer - which would be the largest cash acquisition on record - comes amid a wave of mergers in the agriculture sector.
The drop in commodity prices has put pressure on companies such as Monsanto, with farmers' cutting orders for supplies.
A Bayer takeover of Monsanto is likely to draw close scrutiny from anti-trust regulators because of the sheer size of the combined company and the control it would have over the global seeds and sprays markets.
Farming groups have raised concerns that such mergers could lead to fewer choices and higher prices while opponents of genetically modified food in Europe worry about Monsanto's influence on the continent.
Combining Bayer and Monsanto would make it the market leader in the US, Europe and Asia.
Bayer's farm business produces seeds as well as chemicals to combat weeds and insects, but it is better known for its healthcare products such as Aspirin and Alka-Seltzer.
Monsanto is primarily known for its genetically modified seeds for crops including corn, soybeans, cotton, wheat and sugar cane. Such seeds have attracted criticism from some environmental activists.