US authorities are stepping up their probe of Wall Street firms' hiring practices abroad, sending letters seeking information from major financial firms including Morgan Stanley and Citigroup, according to people familiar with the matter.

JP Morgan Chase in August disclosed it is the subject of criminal and civil investigations by the Justice Department and the Securities and Exchange Commission over hires made in China. The SEC's civil probe has since expanded to include other big Wall Street firms. JP Morgan has declined to comment on the filing.