Sources close to Monsanto have told the news agency Reuters that Monsanto is putting together a new offer for Swiss rival Syngenta.

The news agency said it understands the US agrochemical and GMO giant is still working on a proposal after Syngenta rejected its $45 million unsolicited bid, which it said undervalued the company.

A successful takeover of Syngenta would give Monsanto, which dominates the market for seeds and genetically-modified crops, access to the Swiss company's lucrative crop protection chemicals portfolio and create an industry behemoth with combined sales of more than $31 billion.

Major investors in Syngenta are said to have expressed confidence that Syngenta will give up its resistance if the US group ups the ante by at least 10% more. A banker commented to Reuters, however, that Monsanto might need to pay a premium of up to 40% to make the deal attractive to the Basel firm's shareholders.

To ease antitrust concerns, reports say Monsanto could also team up with an industry partner to acquire Syngenta's US seeds business. Both two groups are already seen as market leaders in the American seeds industry, so that Monsanto alone would not be able to clinch a deal.