The Pentagon says the U.S. is not providing airstrikes in support of Iraq's ground offensive against Islamic State fighters in the northern city of Tikrit because the Iraqi government did not request it.

A Pentagon spokesman, Army Col. Steve Warren, said the U.S. was alerted to the offensive before it started Monday but was not asked to provide air power.

Warren said he could not comment on any Iranian military involvement, beyond saying Washington is aware of Iran's interest in Iraq's fight against the Islamic State group that controls key territory, including Tikrit and Mosul.

Warren said he could not provide an assessment of Iraqi progress in the Tikrit battle.