The new deployment is the third such boost in US troop levels in Iraq since April, underscoring the difficulties President Barack Obama has had in extracting the US military from the country.
Iraqi Prime Minister Haider al-Abadi said in a statement that his government asked for more US military trainers and advisers. Obama called it a "somber decision."
"I've always been very mindful that when I send any of our outstanding men and women in uniform into a war theater, they're taking a risk that they might not come back," Obama said during a town hall event at a military base in Fort Lee, Virginia, televised on CNN.
The new troops will train and advise Iraqi security forces and Kurdish peshmerga forces, primarily in the Mosul fight, but also serve "to protect and expand Iraqi security forces' gains elsewhere in Iraq," US Defense Secretary Ash Carter said.
"We've said all along - whenever we see opportunities to accelerate the campaign, we want to seize them," Carter said.