WASHINGTON (AFP) - President Barack Obama on Tuesday ordered about 350 more US troops to Baghdad to protect American diplomatic facilities and staff in the Iraqi capital, the White House said.

The announcement came hours after the so-called Islamic State, also known as the Islamic State in Iraq and Levant (ISIL), released another video showing a masked militant with a British accent cutting the throat of a US captive.

The Sunni extremist group has declared an Islamic caliphate in regions under its control in Iraq and Syria, after it swept through much of the Sunni Arab heartland north of Baghdad and then stormed minority Christian and Yazidi areas.

The US has been carrying out air strikes on IS positions in Iraq and already has hundreds of personnel shoring up diplomatic security in Iraq.

"The president authorised the Department of Defence to fulfill a Department of State request for approximately 350 additional US military personnel to protect our diplomatic facilities and personnel in Baghdad, Iraq," a White House statement said. "We will (also) continue to support the government of Iraq's efforts to counter ISIL, which poses a threat not only to Iraq, but to the broader Middle East and US personnel and interests in the region."

Obama, on his way to Estonia and a Nato summit in Wales, "will be consulting this week with Nato allies regarding additional actions to take against ISIL and to develop a broad-based international coalition to implement a comprehensive strategy to protect our people and to support our partners in the fight against ISIL," the White House said.

The latest troop movement "builds upon previous embassy security deployments announced on June 15 and June 30 and will bring the total forces responsible for augmenting diplomatic security in Iraq up to approximately 820," said Pentagon Press Secretary Rear Admiral John Kirby.