CNOOC’s attempted takeover of Unocal was on Monday night dealt a setback as US lawmakers passed an amendment that would delay a regulatory review of the Chinese company’s proposed deal by months.

The amendment in effect bars the Committee on Foreign Investments in the US (CFIUS), which reviews foreign takeovers of US companies, from ending its investigation of the proposed deal until three weeks after a government study of the effect of China’s energy needs on US security and the trade relationship with China.