China’s customs authorities have increased inspections of U.S. agricultural imports just as trade tensions between the world’s two largest economies intensify, according to people with knowledge of the matter.

Inspections are taking longer than usual and procedures are more complicated, according to the people, who asked not to be identified because they aren’t authorized to speak to the media. Quarantine authorities have strengthened inspections of products including fruit, pork and lumber, according to the people. Calls to the office of the spokesman of General Administration of Customs weren’t immediately answered.

The greater scrutiny comes after China’s General Administration of Customs said earlier this week that it had strengthened inspections of U.S. apples and wood logs at major Chinese ports after pests and diseases were found in some shipments. Substandard imports may be returned or destroyed, according to a notice published May 7 on its website.

“It is a kind of invisible trade war, which will give China more bargaining power in negotiations” with the U.S., Wang Zhong, chief consultant with China-based Systematic, Strategic & Soft Consulting Co., said in an interview. Farm products are often targeted by the government in dealing with bilateral relations, Wang added.

The inspections don’t just affect agriculture. Ford and premium Lincoln-branded cars have also faced delays at China customs, with officials asking for extra technical checks, Reuters reports, citing unidentified people familiar with matter.