US authorities said Friday they had finalized new rules for a controversial review program on foreign investment in American companies for potential national security threats.

The rules retain the Committee on Foreign Investment in the United States (CFIUS) but with formal time limits and notification requirements for Congress.

The revision `reaffirms America`s longstanding policy of openness to investment, consistent with the protection of our national security,` said Treasury Secretary Henry Paulson.

`These regulations reflect CFIUS`s careful consideration of all comments submitted during the public comment period.`

An initial review must be completed within 30 days, `to determine the effect of the transaction on national security and to address any national security concerns,` according to the rules released by officials.

An additional investigation must be concluded in most cases within 45 days.

Under the rules, an acquisition by a foreign person holding 10 percent or less of a US firm would not be reviewed if the investment is `solely for the purpose of investment.`

The rules also define `critical infrastructure` for the nation for which a review may be carried out.

The rules implement a law passed amid an outcry in Congress over Dubai Ports World`s planned acquisition in 2006 of a company managing US port facilities, which prompted DPW to back away from the deal.

The CFIUS review process, which up to now had been by executive order of the president, affects only a small percentage of foreign investments but has been used to block some controversial acquisitions.

This year, a planned buyout of US technology firm 3Com by a private equity firm and and China`s Huawei Technologies was scrubbed because of what the companies claimed was opposition by CFIUS.

CFIUS is an interagency panel led by the Treasury that operates in secrecy. It also includes the heads of agencies including Homeland Security, Commerce, Energy, Defense and State, and the attorney general.