Iraqi soldiers take position in a street of Ramadi during a military operation against anti-government fighters. (Azher Shallal/AFP/Getty Images)

Iraqi forces faced fierce resistance as they launched an offensive Sunday to vanquish pockets of al-Qaeda-linked militants in the city of Ramadi, underscoring the resilience of insurgents, who have also dug in further east in Fallujah.

With the Iraqi armed forces lacking expertise and equipment, the government’s struggle to secure the key cities has stoked concerns that the unrest will spread.

In an unnerving sign, Baghdad was put on lockdown Saturday night amid a prison break, car bombings and mortar fire. Security officials said the violence appeared to involve the al-Qaeda-linked Islamic State of Iraq and Syria, or ISIS — the same force that seized Ramadi and Fallujah early this month.

The group’s leader, Abu Bakr al-Baghdadi, urged his followers in an audio recording Sunday to launch attacks on Shiites in Baghdad and in southern Iraq.

The Obama administration has been alarmed by the ISIS fighters’ success in Ramadi and Fallujah, a city where only a few years ago U.S. forces fought their bloodiest battle since the Vietnam War. U.S. authorities have vowed to speed up deliveries of weapons to the Iraqi government, including small arms. American officials are pushing Prime Minister Nouri al-Maliki to pass the weapons on to tribesmen battling alongside his government’s forces in Ramadi.

Still, those tribesmen say arms have been slow to arrive.

Iraqi military officials had initially predicted that they would regain control of Ramadi within a few days of its capture. But forces aligned with the Shiite-dominated government have been unable to consolidate control of the two cities, which are in mostly Sunni Anbar province.

On Sunday, helicopter gunships, pro-government tribesmen, Iraqi special forces and police pursued the remaining militants and rebel tribesmen in Ramadi, 60 miles west of Baghdad, which was sealed off and under curfew. But they were forced to retreat after suffering losses, residents said.

Ahmed Abu Risha, a prominent tribal leader who sides with the government, said the worst of Sunday’s fighting in Ramadi had occurred around the sports stadium, where insurgents were holed up. He said he lost no men. However, Anbar officials cited by the Associated Press said 20 police officers and pro-government tribesmen had been killed or injured in the offensive.

A local journalist, who spoke on the condition of anonymity for security reasons, said Abu Risha’s men and security forces had withdrawn after suffering casualties. Four of their Humvees, two tanks and an armored truck were destroyed, the journalist said.

Fallujah, meanwhile, has had only limited skirmishes in recent days, largely in the east and the northwest. The city is held by a loose alliance of fighters from ISIS, other Islamist groups, rebel tribesmen and combatants loyal to the Baath Party of former president Saddam Hussein, who was a Sunni. Baghdad fears that a ground assault might increase the government’s already deep unpopularity in the city, so its efforts to oust the insurgents have focused on negotiations.

“If the government was serious about wanting to support the tribes, they could clean up in three days,” said a government official involved in security issues, who spoke on the condition of anonymity because of the sensitivity of the topic. “All [the tribes] need is fuel, ammunition and medium-sized weapons.”

But the alliance between the government and the tribes is a shaky one. Some of the Sunni tribal members had spearheaded a protest movement against the government that resulted in demonstrations and sit-ins in Anbar last year.

“The government is worried that if they give the tribes weapons in the day, the same weapons will be used to shoot them at night,” the official said.

He said intelligence indicates that just 120 ISIS fighters took Ramadi, while about 200 secured Fallujah, though they were joined by some fighters from loosely affiliated “sleeper cells.”

In an interview Thursday, Maliki said most of the tribes are backing the government in the fight, though some are split. In addition to requesting more arms to battle the al-Qaeda-linked forces, he reached out for U.S. support on counterterrorism training for Iraqi forces. On Sunday, Jordan said it would be willing to host such training.

Maliki has repeatedly accused outside actors of supporting the rise of ISIS, apparently referring to majority-Sunni countries in the region.

However, Maliki’s political rivals accuse the Shiite prime minister of failing to build ties with Iraq’s Sunni minority, which they say allowed al-Qaeda to take advantage of growing disenfranchisement within the community.

Indeed, it was after Iraqi security forces dismantled a protest camp and raided a Sunni lawmaker’s house in Ramadi, killing his brother, that the army was forced to withdraw from the Anbar cities in the face of deadly clashes, giving ISIS the space to take hold.

The government official said ISIS has achieved its main goal.

“It wasn’t and it never has been its priority to take land,” he said. “They freed their prisoners, they robbed the banks, and they got the headlines. They already won. They’ll leave their pawns to fight it out.”