Iraqi Prime Minister Nuri al-Maliki secretly asked U.S. President Barack Obama's administration to consider airstrikes against militant staging areas as the threat from Sunni insurgents mounted last month, the New York Times reported on Wednesday, citing U.S. and Iraqi officials.

The Times quoted American experts who visited Baghdad earlier this year as saying they were told that Iraqi leaders hoped American air power could be used to hit the militants' staging and training areas inside Iraq, and help Iraq's forces stop them from crossing into the country from Syria.

The Wall Street Journal on Wednesday quoted senior U.S. officials as saying Iraq had signaled it would let the United States strike Al-Qaida militant targets in Iraq with manned aircraft or drones.

"We are not going to get into details of our diplomatic discussions but the government of Iraq has made clear that they welcome our support" against the militants, White House national security council spokeswoman Bernadette Meehan said on Wednesday.

"We have expedited shipments of military equipment since the beginning of the year, ramped up training of Iraqi Security Forces, and worked intensively to help Iraq implement a holistic approach to counter this terrorist threat," Meehan said in a statement. "Our assistance has been comprehensive, is continuing and will increase."

Sunni rebels from an Al-Qaida splinter group overran the Iraqi city of Tikrit on Wednesday and closed in on the biggest oil refinery in the country, making further gains in their rapid military advance against the Shi'ite-led government in Baghdad.

The threat to the Baiji refinery came after militants from the Islamic State in Iraq and the Levant, or ISIL group, seized the northern city of Mosul, advancing their aim of creating a Sunni Caliphate straddling the border between Iraq and Syria.

The fall of Mosul, Iraq's second-biggest city, is a blow to attempts to defeat the militants, who have seized territory in Iraq over the past year following the withdrawal of U.S. forces.