Syngenta turned down a $47-billion bid from Monsanto in August and has reportedly backed away from a $41.6-billion offer from China National Chemical (ChemChina).
Syngenta and ChemChina were said to still be in talks, but Monsanto's chief operating officer Brett Begemann told reporters that the failure of those two sides to strike a deal had spurred Monsanto to rethink another offer.
"If you are asking me are we having conversations inside of Monsanto, well of course we are," Bloomberg quoted Begemann as saying.
"Talking and doing is two different things."
Syngenta's shares were up by 1.8 percent on the news at 1130 GMT, trading at 382 Swiss francs ($376.70, 352.80 euros). Switzerland's main SMI index was largely flat.
Syngenta declined to comment on the rumours that the US seed giant might make another offer.
Syngenta saw its shares enter a tailspin in August when Monsanto abandoned its takeover bid, but tried to calm angry shareholders by announcing a $2-billion share buy-back deal in September.
The Swiss company reportedly rejected the ChemChina offer due to regulatory concerns.