Special Counsel Robert Mueller said he lacked confidence to clear Donald Trump of obstruction of justice but suggested Congress could take action on at least 10 instances where the president sought to interfere with the probe.

"We concluded that Congress has authority to prohibit a president's corrupt use of his authority in order to protect the integrity of the administration of justice," he said in the report sent to Congress on Thursday.

Mueller said acts of possible obstruction include "discouragement of cooperation with the government and suggestions of possible future pardons." The 448-page report cited actions including Trump's firing of FBI Director James Comey and efforts to have former Attorney General Jeff Sessions take control of the investigation.