Washington: Federal prosecutors are investigating whether private US security firm Blackwater USA shipped unlicensed automatic military weapons into Iraq, a newspaper reported on Saturday.

Two former Blackwater employees have pleaded guilty to weapons charges and are cooperating with the investigation, The News & Observer reported.

The newspaper quoted unnamed sources as saying that prosecutors are probing whether Blackwater was shipping unlicensed weapons, night-vision scopes and gun kits.

Federal prosecutors in North Carolina are handling the case, The News & Observer reported. US Justice Department officials were not immediately available to comment.

A deadly shooting involving Blackwater security guards on Sunday caused uproar in Baghdad after 11 people were killed. Iraq and the US are investigating the incident.

Blackwater officials said that their guards were just responding to armed attack on a US convoy, but Iraqi officials said the guards indiscriminately fired at civilians.

Also on Saturday, The Washington Post reported that Iraq's investigation into Sunday's shootings has expanded to include allegations about Blackwater's involvement in six other violent incidents this year that left at least 10 Iraqis dead.