Kirkuk Outskirts
A man drives a vehicle (green plate) belonging to the Iraqi army at a Kurdish military checkpoint on the outskirts of the northern Iraqi city of Kirkuk on June 11, 2014. Kurdish forces initially attempted to stop people driving vehicles looted from the Iraqi army at the checkpoint but eventually let them through as there were too many of them. Sunni insurgents from an al Qaeda splinter group extended their control from the northern city of Mosul on Wednesday to an area further south that includes Iraq's biggest oil refinery in a devastating show of strength against the Shi'ite-led government. Reuters/Ako Rasheed

Update as of 4:47 a.m. EDT: Kurdish forces reportedly took over full control of the northern city of Kirkuk, reports said, after Iraqi troops abandoned their posts. Kirkuk has strategic value because it is home to some of the country's main oilfields.

"The whole of Kirkuk has fallen into the hands of peshmerga," Jabbar Yawar, a spokesman for the Kurdish forces told Reuters. "No Iraq army remains in Kirkuk now."

Kurds were reportedly driven away from Kirkuk during Saddam Hussein's regime, according to the BBC, and the region's oil wells were subsequently controlled by the Iraqi government. The region is a source of dispute between Iraq's Arabs and the local Kurdish population.

In an audio recording released early Thursday by the Islamic State of Iraq and Levant, or ISIL, the militant group promised to seize more cities in Iraq, including the capital Baghdad, CNN reported.

In a 17-minute-long audio recording, reportedly in the voice of spokesperson Abu Mohammed al-Adnani, ISIL said, according to CNN, which could not confirm the authenticity of the message, that the group will continue fighting to gain control over more cities in Iraq, including Baghdad and Karbala, a city sacred to the country's Shia population. The recording’s release follows the group’s siege and capture on Wednesday of Tikrit, which lies about 87 miles from Baghdad, and is the home town of former dictator Saddam Hussein.

“Continue your march as the battle is not yet raging," the voice said in the recording, according to CNN. “It will rage in Baghdad and Karbala. So be ready for it.”

The attack has been condemned by various leaders worldwide, including the U.S. and UK, while the United Nations has expressed deep concern for the safety of the people in the region, who have started fleeing the cities. Following the attack on Mosul earlier this week, which led to more than 500,000 people evacuating the city, ISIL has taken control over Tikrit, which is home to major oil refineries that have shut down since the siege.

Meanwhile, the U.S. said that it will “stand ready” to assist the Iraqi government, which has reportedly showed willingness for allowing the U.S. to halt the militant group's advance with the help of drone strikes or manned aircraft bombers.

“The security reiterated the United States commitment to working with the Iraqi Government and leaders across Iraq to support a unified approach against ISIL’s continued aggression. We are in touch with the governments of Turkey and Iraq, and stand ready to provide any appropriate assistance,” Jen Psaki, spokesperson for the State Department said, in a briefing Wednesday, adding: “You can expect that we will provide additional assistance to the Iraqi Government to combat the threat from ISIL.”

Psaki added that Brett McGurk, an adviser to the U.S. ambassador to Iraq, had spoken to various leaders in Iraq including Prime Minister Nouri al-Maliki, and the UN’s representatives in the region, to discuss ways in which the U.S. could help the country's government.