* DuPont says Monsanto patents are invalid

* DuPont shares fall 2.3 pct, Monsanto down 3.9 pct (Adds Monsanto comment)

NEW YORK, June 16 (Reuters) - Chemicals maker DuPont DD.N launched a counter-lawsuit against rival seedmaker Monsanto Co MON.N on Tuesday to force the company to allow it to sell its own products with Monsanto's genetically modified soybeans.

The lawsuit is the latest fight in the battle for the rights to new biotechnology that could be worth billions of dollars in sales to farmers worldwide who are seeking boost crop yields and profits.

DuPont’s suit, filed in the U.S. District Court in St. Louis, comes a month after Monsanto sued DuPont’s Pioneer Hi-Bred International business, contending it had breached its license to market Roundup Ready soybean and corn seeds that are modified to be more tolerant of herbicides.

In a statement, DuPont said it is allowed to combine, or “stack,” the genetic traits of its Optimum GAT technology with Roundup Ready under its rights in a licensing agreement with Monsanto, and that Monsanto’s patents around its soybean traits were invalid.

“Monsanto’s lawsuit is another tactic used to restrict the availability of competitive products,” DuPont Vice President James Borel said in a statement.

Monsanto, the world’s biggest seedmaker, has said Pioneer has misused the Roundup Ready traits to mask problems with its own Optimum GAT technology.

“Rather than tell the truth about its failed product, DuPont improperly used Monsanto’s proven technology to cover up the flaws of Optimum GAT. This is the real issue in the case,” Scott Partridge, Monsanto’s deputy general counsel, said in a statement in response to DuPont’s claim.

Earlier this month, DuPont and Germany's BASF BASF.DE sued each other of claims of patent infringement for the technologies used in the DuPont's Optimum GAT.

On the New York Stock Exchange, Monsanto closed off 3.9 percent at $81.62 and DuPont fell 2.3 percent to 25.20.

The case is: Monsanto v Dupont, 09-cv-00686.