The nearly $257 million purchase of Waltham battery maker A123 Systems Inc. by a Chinese conglomerate hangs on approval by an obscure, secretive federal panel that scrutinizes foreign acquisitions of American businesses to protect national security.

The panel, known as the Committee on Foreign Investment in the United States, or CFIUS, has approved nearly all the deals it has considered over the past several years, according to committee annual reports. But more recently it has intensified its scrutiny of foreign entities — particularly those from China — seeking to obtain advanced US technologies, presenting a hurdle for Wanxiang America Corp. as it tries to acquire nearly all the assets of bankrupt A123.