Bottles of Roundup weed killer move along the production line at the herbicide manufacturing facility operated by Monsanto Co. in Antwerp, Belgium.

German pharmaceutical and crops manufacturer Bayer and Monsanto are closing in on a deal, with Monsanto telling Bayer that it is "close" on price, sources told CNBC Tuesday.

The deal could happen within a week or two, people familiar with the matter say.

On Monday, Bayer said that its negotiations with Monsanto had advanced, and it was now willing to offer more than $65 billion to acquire the world's largest seed company.

Bayer's announcement came as the gap in price expectations between the two companies has narrowed significantly, although important terms, including potential divestitures in case of antitrust scrutiny, have yet to be agreed on.

However, one leading shareholder in Monsanto, John Bennett, director of European equities at fund manager Henderson Global Investors, said he opposed the revised terms of the offer, Reuters reported.

Bayer in a statement said that it was prepared to offer $127.50 per share in connection with a negotiated deal, up from its previous offer of $125 per share. The Bayer statement confirmed a report by German daily Rheinische Post earlier on Monday.

Rheinische Post also reported, citing sources which it did not identify, that an offer of $130 per share may be necessary to clinch a deal with Monsanto "in a swift and friendly way."

Bayer's bid was already the largest all-cash proposed takeover on record. A deal with Monsanto would give the German company a shot at grabbing the top spot in the fast-consolidating farm supplies industry.