U.S.-based Monsanto increased its offer to buy Syngenta to around $47 billion, or 470 Swiss francs ($503.70) per share from 449 francs per share previously, a person familiar with the matter said on Monday.

The stock had closed at 357.60 francs on Monday.

Syngenta officials did not return a request for comment on Tuesday morning about whether a sweetened deal from Monsanto would convince them to agree to discuss a transaction. It had declined comment late on Monday.

Syngenta had rejected a previous offer and has refused to open its books to its rival.

The new offer includes an increase in the break-up fee to $3 billion from $2 billion if the transaction is blocked by regulators or falls apart for other reasons, the person said.

Monsanto is aiming to combine its world-leading seeds business with Syngenta's own seeds and pesticides operations, contending the deal will make both firms more efficient by developing seeds and pesticides in tandem and integrating sales and distribution strategies.

Syngenta has so far argued the deal faces tough regulatory hurdles that Monsanto has not addressed and that the 449 franc offer undervalues the company.

"The biggest concern to Syngenta seems to be that the proposed follow-on disposal of Syngenta’s seeds business and overlapping herbicides is (tackling) anti-trust issues

from a horizontal perspective only," Merrill Lynch wrote in a research note after reports on Monday of the higher offer.

(Reporting by John Miller; Editing by Michael Shields)