In Palmyra, the Syrian air force launched strikes at buildings captured by Islamic State, whose occupation of the city has raised fears that the insurgents will destroy its famed Roman ruins.
Islamic state has killed at least 217 people execution-style, including children, since it moved into the Palmyra area 10 days ago, according to the British-based Syrian Observatory for Human Rights. A further 300 soldiers were killed in fighting before the city was captured, according to the monitoring group.
Islamic State reinforced Ramadi on Monday, deploying fighters in preparation for battle against security forces and paramilitary groups advancing on the provincial capital.
Iraqi forces have regained ground east of the city since launching a counter-offensive on Saturday, a week after it was overrun by the insurgents, and on Monday retook a rural area south of the city.
Police sources said Iraqi security forces supported by Iran-backed Shi'ite militia groups and locally recruited Sunni tribal fighters had retaken parts of al-Tash, 20 km (12 miles) south of Ramadi, which lies only a short distance from Baghdad.
Ramadi residents said trucks carrying Islamic State fighters arrived on Sunday evening before spreading out across the city.
Local man Abu Saed heard a commotion outside his house in the city's southeastern Officers neighborhood. "I saw two trucks pull up outside with dozens of fighters carrying arms running quickly into nearby buildings and taking cover."
Another resident said at least 40 fighters had jumped out of three trucks that arrived in the southern al-Tamim district at around 8 pm on Sunday. "They were carrying weapons and wearing mostly khaki dress with ammunition belts wrapped around their chests," said Abu Mutaz. "They were talking in an Arabic dialect, they were not Iraqis."
The fall of Ramadi is the most significant setback for Iraqi forces in almost a year and has cast doubt on the effectiveness of a U.S.-led strategy for fighting the Sunni militant group.
The seizures of Ramadi and Palmyra were the group's biggest successes since a U.S.-led coalition launched an air war against it last year.
The near simultaneous victories against the Iraqi and Syrian armies have forced Washington to examine its strategy, which involves bombing from the air but leaving fighting on the ground to local forces.
In a sharp criticism of Washington's ally, U.S. Defense Secretary Ashton Carter accused the Iraqi army of abandoning Ramadi to a much smaller enemy force. "The Iraqi forces just showed no will to fight," he told CNN's State of the Union program on Sunday.
The general in charge of Iran's paramilitary activities in the Middle East meanwhile said the United States and other powers were failing to confront Islamic State.
"Today, in the fight against this dangerous phenomenon, nobody is present except Iran," said Major General Qassem Soleimani, who is often seen on the battlefields of Iraq.
But in a move that could mark an expansion of U.S. involvement in the conflict, Turkey said it and the United States had agreed in principle to give air support to some forces from Syria's mainstream opposition.
Days after taking Ramadi, Islamic State also defeated Syrian government forces to capture Palmyra, home to 50,000 people and site of some of the world's most extensive and best-preserved Roman ruins.
The Sunni Muslim militants have proclaimed a caliphate to rule over all Muslims from territory they hold in Syria and Iraq. They have carried out mass killings in towns and cities they have captured, and destroyed ancient monuments, which they consider evidence of paganism.
In Syria, Hezbollah fighters captured two hilltops from al Qaeda's Syria wing Nusra Front in areas close to the Lebanese border and killed dozens of enemy combatants, Hezbollah-run al-Manar television reported on Monday.
Iranian-backed Hezbollah has backed Syrian President Bashar al-Assad in his country's civil war. The group's leader, Sayyed Hassan Nasrallah, has vowed to clear the border area of Sunni Muslim militant groups that have carried out attacks on Lebanese soil.