The head of US intelligence said Thursday that China is "the leading suspect" in a massive data breach of Washington's government personnel files, but that an investigation is ongoing.

The comments from James Clapper, the director of national intelligence, were the first on the record remarks by an official implicating Beijing in the hack affecting millions of US government employees and others.

Clapper told a Washington intelligence conference that "China remains the leading suspect" but that "the US government continues to investigate" the hack, according to his office, which confirmed earlier media reports.

Since the breach was made public earlier this month, US officials have said privately they believe China was involved, and cybersecurity specialists have said the breach appeared to be part of an effort to build a database that could be used for espionage purposes.

The US Office of Personnel Management has said more than four million employees, retirees, contractors and job applicants may have had their personal data compromised in the breach, and that a second incident will boost that figure.

Some reports have said the number may be 14 million or higher, but officials have said the total is still being determined.

Amid an outcry among employees and lawmakers, the administration has announced new steps to boost the security of its online records, some segments of which are using decades-old technology.