Smithfield Foods Inc, the world's largest pork processor, said Wednesday that a United States foreign-investment committee will conduct an additional 45-day review of its proposed $4.7 billion takeover by Shuanghui International Holdings Ltd, China's largest meat producer.

A Smithfield statement announcing the secondary review offered no comment on the reason for it. But some experts said following an initial 30-day review, the Committee on Foreign Investment in the United States, or CFIUS, which vets proposed foreign acquisitions of US companies for national security concerns, is conducting the additional scrutiny due to "political pressure".

"Moving to the second phase review of this transaction is clearly in response to political pressure exerted by Senate hearings on the takeover earlier in July," said Cliff Burns, a Washington-based attorney with Bryan Cave LLP.