China's biggest ever foreign takeover is taking some heat in the U.S.

State-owned ChemChina last month announced a $43 billion deal to buy Switzerland's Syngenta, a giant seed and pesticide company. But the planned acquisition has set off alarm bells in the U.S., where Syngenta supplies farmers and runs research and production facilities.

Four U.S. senators called Thursday for the Committee on Foreign Investment in the United States to review the deal "with a specific focus on the potential effects on food security and the safety of our food system."

One of the senators, Chuck Grassley of Iowa, told a local radio station earlier this week that the deal could create a conflict of interest, with the Chinese government both regulating biotechnology products and owning a company that makes them. Syngenta's products, which are aimed at helping farmers get more out of their land, include genetically modified seeds for crops like corn, soybeans and sugar beet.

Related: ChemChina offers $43 billion for Switzerland's Syngenta

ChemChina disputed the claims and said it welcomes a full review by the U.S. government.

"This transaction is crucial to maintaining choice and competition because it enables Syngenta to continue as a leading innovator in seeds, traits and crop protection products, which will preserve the safety, reliability and diversity of the global food supply," the company said in a statement.

U.S. Secretary of Agriculture Tom Vilsack has already expressed doubts about the deal because of how he says China regulates foreign biotech products.

"I have a watchful eye on all of this and continue to be extremely concerned about the way in which biotechnology and innovation is being treated and impeded by a system in China that often times is not based on science and appears to be more based on politics," Vilsack said last month.

Related: Chinese interest in Chicago Stock Exchange causes alarm in Congress

The Syngenta deal is the latest big Chinese acquisition to face objections in the U.S.

Dozens of lawmakers sent a letter to the Committee on Foreign Investment last month calling for a "full and rigorous investigation" into whether a Chinese company that's planning to buy the Chicago Stock Exchange has a close relationship with China's government.

And the U.S. technology company Western Digital said a month ago that it was calling off a plan to take a $3.8 billion investment from a Chinese firm because the Treasury committee had decided to investigate the deal.

Chinese companies are in the midst of a global buying frenzy. So far in 2016, they have already announced $104 billion in foreign deals, which is 98% of the amount for the whole of last year, according to Dealogic.