The suspect who opened fire on two National Guard soldiers just blocks from the White House is an Afghan national who worked with a CIA-backed unit in Afghanistan, according to officials.

He worked with "the US government, including CIA, as a member of a partner force in Kandahar" during the US-led war in the country, CIA director John Ratcliffe has said.

The suspect, who was wounded in an exchange of gunfire before he was arrested, was identified by the Department of Homeland Security as Rahmanullah Lakanwal.

Attorney general Pam Bondi said the US government plans to bring terrorism charges against the gunman and seek a sentence of life in prison "at a minimum".

"A lone gunman opened fire without provocation, ambush style, armed with a .357 Smith & Wesson revolver," she told reporters.

The two wounded soldiers were a man and a woman, Ms Bondi said.

She identified them as 20-year-old Sarah Beckstrom and 24-year-old Andrew Wolfe. They have undergone surgery and remain in critical condition.

US President Donald Trump, who was at his resort in Florida at the time of the attack, released a prerecorded video statement late on Wednesday calling the shooting "an act of evil, an act of hatred and an act of terror".

He said his administration would "re-examine" all Afghans who arrived in the US during the presidency of his predecessor, Joe Biden.

The US Citizenship and Immigration Services agency has said it has halted processing of all immigration requests relating to Afghan nationals indefinitely, "pending further review of security and vetting protocols".