President Barack Obama on Tuesday ordered about 350 more US troops to Baghdad to safeguard American diplomats in the Iraqi capital, increasing the US military presence to more than 1,000 forces.

The move comes after a request last month from the State Department for additional US troops to bolster security for the US embassy and other facilities in Iraq, where Islamic State extremists have seized territory in the north and west of the country.

White House and Pentagon officials issued the announcement hours after the IS jihadists released another grisly video showing a masked militant with a British accent cutting the throat of an American captive.

President Barack Obama has vowed the US will not be intimidated, after Islamic State militants released a video showing the beheading of American journalist Steven Sotloff.