This has not stopped Premier Li Keqiang's "going out" policy, which encourages Chinese companies to buy foreign trophy assets. The push -- aided by CFIUS's history of rarely shooting down deals altogether -- contributed to Chinese M&A activity in the United States reaching a record high of $32 billion so far this year.

To be sure, CFIUS has approached companies in the past as well, and does not limit its review to only Chinese deals. In 2010, CFIUS contacted Russian internet company CMail.ru and AOL Inc over the latter's $188 million divestment of messaging service ICQ to Cmail.ru, which had already been completed. The CFIUS review in that instance did not require the deal to be unraveled.

On rare occasions, the panel has also vetoed deals, such as the $3.3 billion sale of Koninklijke Philips NV's lighting business to a consortium of Chinese investors, which it blocked last January.

But Ironshore and similar cases this year show that the U.S. watchdog is flexing its muscles in a more subtle, albeit disruptive, fashion.

"Companies may assume that there is no chance that CFIUS would have an interest in their transaction, but that runs the risk of possible miscalculation," said Eric Dinallo, a partner at law firm Debevoise & Plimpton LLP.

RISING SCRUTINY

CFIUS, an agency made up of eight U.S. government departments and chaired by the Treasury Secretary, does not publicize the reasons for its decisions. The majority of transactions involve private companies with no SEC filings.

Recent regulatory filings and statements by publicly listed companies, however, offer glimpses of CFIUS catching some companies off guard. U.S. electronics distributor Ingram Micro Inc said in July it would seek CFIUS approval for its acquisition by Chinese shipping company Tianjin Tianhai Investment, despite saying in February it did not need to, following "consultation" with CFIUS.

As a result, in August Ingram Micro pushed back the deadline for the deal with Tianjin to close by three months to Nov. 13. CFIUS is interested in learning more about the company's supply of technology to the U.S. government, according to the sources. Ingram Micro and Tianjin Tianhai declined to comment.

CFIUS has added staff and resources in the last two years to identify non-notified transactions, the sources said, though the number of additional people recruited or the extra funding it was given could not be learned.