U.S. Treasury Secretary Jack Lew has written to the Chinese government warning that a recent spate of antitrust investigations against foreign companies could have serious implications for U.S.-China relations, the Wall Street Journal reported on Sunday.

The newspaper said Lew sent a letter to Chinese Vice Premier Wang Yang and said that China's recent focus on foreign companies could devalue foreign intellectual property, citing people briefed on the contents of the letter.

The paper said the letter was sent in recent days. Representatives of the Chinese cabinet did not respond to requests for comment, the WSJ reported.

The U.S. Treasury declined comment when asked to confirm the report. "We regularly correspond with our international counterparts on a variety of issues," a Treasury official said earlier.

At least 30 foreign firms, including U.S. companies such as Microsoft Corp and chip maker Qualcomm Inc have come under scrutiny as China seeks to enforce a 2008 anti-monopoly law that some critics say is being used to unfairly target foreign firms.

After July talks on the U.S.-China Strategic and Economic Dialog, Treasury said China "recognized that the objective of competition policy is to promote consumer welfare and economic efficiency, rather than to promote individual competitors or industries, and that enforcement of its competition law should be fair, objective, transparent, and non-discriminatory."

In the last two weeks, four leading international business lobbies have raised alarm over the Chinese investigations.

Their complaints range from worries that foreign companies are being unfairly targeted by probes motivated by China's industrial policy aims to concerns over the use of strong-arm tactics by Chinese regulators.

But China's three anti-monopoly regulators said on Thursday they are not targeting multinational firms and the enforcement work is fair and transparent.

China's Premier Li Keqiang has said that only 10 percent of companies impacted by anti-trust investigations are foreign.

The Qualcomm case could yield record fines of more than $1 billion. Regulators have also announced their first-ever punishments of foreign carmakers for price-fixing, fining a Chinese venture of Volkswagen AG and the China sales unit of Fiat's Chrysler a combined $46 million.

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