Iraqi government forces have taken control of "vast areas" of Kirkuk, including oilfields, according to Iraqi state TV - but the Kurdistan Regional Government has disputed the assertions.
Tensions over the future of Iraqi Kurdistan have been high since Iraqi Kurds voted for independence in a September referendum.
The U.S. Defense Department has urged both sides "to avoid additional escalatory actions", and to concentrate on the battle against the so-called Islamic State.
From Washington, Kate Fisher reports
IN: The Pentagon...'
OUT: SOC
DUR: 36 secs