Syngenta said the offer will now run from May 24 to July 18.

"We don't have all the regulatory approvals yet," a Syngenta spokesman said, adding the company still expected the deal to close by year end.

ChemChina has previously said that the offer period, which initially would have expired on May 23, could be renewed several times for subsequent periods of up to 40 trading days.

People familiar with the matter told Reuters on Monday that the U.S. Department of Agriculture has agreed to join the U.S. government panel that is reviewing the deal.

The move will subject the transaction to additional U.S. government scrutiny and comes after lawmakers called for the USDA be involved in the review so that the impact of the transaction on domestic food security could be better assessed.

Shares in Syngenta, which have traded at a discount to ChemChina's offer of $465 (455 Swiss francs) per share plus a special dividend of 5 francs, were 0.7 percent lower at 390.7 francs at 1355 GMT while the broader Swiss market slipped 0.4 percent.

Switzerland-based traders said the discount would likely persist for now, given the delay. (1 Swiss franc = $1.0218)

(Reporting by Oliver Hirt and Rupert Pretterklieber; Writing by Ludwig Burger; Editing by Keith Weir)