The Iraqi military says extra troops are being sent to Ramadi, a city now largely controlled by the Islamic State (IS) extremist group.
An Iraqi military spokesman told Iraqi state TV on May 16 that the U.S.-led coalition had been supporting Iraqi troops with "painful" air strikes since late on May 15.
The spokesman, Brigadier General Saad Maan Ibrahim, did not give details of the fighting going on in Ramadi, but vowed that the Islamic State militants would be driven from the city "in the coming hours."
On May 16, the militants swept through Ramadi, the capital of Anbar Province.
Later reports said the militants then withdrew from the government building after air strikes forced them to retreat. Unconfirmed reports said the militants left the buildings booby-trapped or on fire.
The assault on Ramadi marked a significant setback for the Iraqi government in its long fight to dislodge militants, who hold about a third of the country.
Late on May 15, U.S. Vice President Joe Biden spoke by phone with Iraqi Prime Minister Haidar al-Abadi about the Islamic State attack on Ramadi.
According to the White House, Biden pledged that the United States would expedite U.S. military aid to Iraqi forces.
"This will include delivery of heavy weaponry, including AT-4 shoulder-held rockets to counter vehicle-borne improvised explosive devices, additional ammunition, and supplies for Iraqi forces," the White House said in a statement.
Earlier, the Pentagon played down the Islamic State gains.
A spokesman told a news conference that the militants were largely "on the defensive" in Iraq.
He said Iraqi forces still controlled most "key facilities, infrastructure and lines of communication" in the Ramadi area.
Ramadi is one of the few towns and cities to have remained under government control in the vast desert Anbar Province, which borders Saudi Arabia, Syria and Jordan.
More than 130,000 people have fled Anbar since the militants renewed their offensive on Ramadi in April, according to the International Organization for Migration.
Reports on the ground said people who had stayed were trying to flee on May 15, but found themselves trapped.
Syrian Forces Battle IS Near Palmyra
Meanwhile, in Syria, heavy fighting between IS militants and the Syrian Army was reported on May 16 in the historic city of Palmyra.
According to the Syrian Observatory for Human Rights, IS has taken control of most of the northern parts of the city. The UK-based group said 13 fighters were killed in ongoing clashes near the Islamic citadel in the city's west.
Earier, the group said that Islamic State militants had executed 23 people on May 15 including nine minors and five women in areas seized from state control outside the city.
It marked the second mass execution reported since Islamic State advanced this week into the area, some 240 kilometers northeast of Damascus.
In the first, the Observatory said the jihadists executed 26 men, beheading 10 of them.
The Islamic State offensive in central Syria has added to the pressures confronting government forces, who have faced significant setbacks since late March in the four-year-long conflict there.
With reporting by AP, AFP, and Reuters