Iran promised vengeance after a U.S. air strike in Baghdad on Friday killed Qassem Soleimani, Tehran's most prominent military commander and the architect of its growing influence in the Middle East.
The overnight attack, authorized by President Donald Trump, was a dramatic escalation in a "shadow war" in the Middle East between Iran and the United States and its allies, principally Israel and Saudi Arabia.
The officials, speaking on condition of anonymity, said the troops would be joining the roughly 750 forces that were sent to Kuwait earlier this week.
U.S. officials told Reuters earlier this week that thousands of additional troops could be sent to the region and had been told to prepare to deploy.
Since May, the United States has already dispatched about 14,000 additional troops to the Middle East.
Reuters first reported on Friday that U.S. Defense Secretary Mark Esper has canceled plans to take personal leave later in January amid rising tension with Iran.
The attack followed a sharp increase in U.S.-Iranian hostilities last week when pro-Iranian militiamen attacked the U.S. embassy in Iraq following a U.S. air raid on the Kataib Hezbollah militia.Iraq's prime minister said with Friday's attack Washington had violated a deal for keeping U.S. troops in his country.