By Greg Roumeliotis

(Reuters) - The U.S. Department of Agriculture has agreed to join the U.S. government panel that is reviewing state-owned ChemChina's planned $43 billion acquisition of Swiss seeds and pesticide maker Syngenta AG, people familiar with the matter said on Monday.

The move will subject the deal to additional government scrutiny. It comes after lawmakers wrote to Treasury Secretary Jacob Lew in March to ask that the USDA be involved in the review so that the potential impact of the transaction on domestic food security could be better assessed.

Lew chairs the Committee on Foreign Investment in the United States (CFIUS), which reviews deals for potential national security threats and is comprised of representatives from 16 U.S. agencies including Treasury, Homeland Security and Defense.

The sources asked not to be identified because the USDA's role in CFIUS has not been publicly disclosed.

Treasury declined to comment, with a spokesperson saying, "By law, information filed with CFIUS may not be disclosed by CFIUS to the public. Accordingly, the department does not comment on information relating to specific CFIUS cases, including whether or not certain parties have filed notices for review."

A USDA spokeswoman also declined to comment, citing the same reasons.

Syngenta declined to comment. A spokesman for China National Chemical Corp, known as ChemChina, did not immediately respond to a request for comment.

While the USDA is not one of the agencies comprising CFIUS, there is precedent of it joining in a review of a deal. In 2013, CFIUS cleared the way for China-based Shuanghui International Holdings Ltd to buy U.S. meat company Smithfield Foods Inc with the USDA's participation.

Syngenta said earlier this year it would make a voluntary filing with CFIUS for its deal with the Chinese state-owned firm "even though no obvious national security concerns were identified during due diligence."

Unveiled in February, the deal is the largest foreign acquisition ever by a Chinese company, as China is looking to secure food supplies for its population.

Syngenta, which is headquartered in North Carolina and generates nearly a quarter of its revenue from North America, is the biggest seller of pesticides in North America and also a key player in seeds. It has other facilities in North Carolina, as well a presence in California, Delaware, Iowa and Minnesota, among other states.

(Reporting by Greg Roumeliotis in New York; Additional reporting by PJ Huffstutter in Chicago; Editing by Leslie Adler)