ISTANBUL -- As the Islamic State group Sunday captured full control of Ramadi, routing Iraqi government forces supported by American airstrikes, the Sunni militants appeared poised to expand their territory. ISIS forces were on the verge of dominating Anbar province, with effective authority over a lucrative trade route connecting its territory in Syria to the outskirts of Baghdad.

Beyond the territorial gains, the fall of Ramadi resonates as a blow to the strength of the Iraqi government and its capacity to marshal an effective offensive against the Islamic State group, also known as ISIS. It heightens concerns about the Obama administration’s strategy in seeking to contain ISIS: The extremists managed to take Ramadi despite an American aerial bombardment that first started in October.

The loss of Ramadi challenged Iraqi government assurances it will soon mount an effective offensive to recapture Mosul, the country’s second-largest city, and a major strategic asset near the Syrian border. The United States and Iraq have both outlined plans to retake Mosul from ISIS as a landmark in their shared effort to defeat the jihadist army. In February the U.S. said as many as 25,000 Iraqi troops were expected to deploy to Mosul in that campaign.

Ramadi was supposed to be an easy prelude, especially following the Iraqi victory in Tikrit in April. Now that this operation has ended in defeat, few analysts give credence to the notion that American airpower plus Iraqi troops will be enough to deliver victory in the higher-stakes theater of Mosul.

For months, U.S. and Iraqi military officials have been debating whether, following the battle of Tikrit, to focus on Anbar or Mosul, said Mike Knights, an expert on Iraq at the Washington Institute for Near East Policy, a Washington think tank. The fall of Ramadi, he said, will hold up the fight that was planned for Mosul.

"The fall of Ramadi was really the nail in the coffin for the idea that we would try and take Mosul first," he said. "Mosul is going to recede even farther into the background now." Knights said the Iraqi military will most likely regain at least some of the ground it lost in Ramadi within the week, because Anbar, more so than any other Sunni region in Iraq, is a place where Shiite militias "know they are guests."

Ramadi finally fell after months of drawn-out battles with Sunni tribesmen in the area. Given that support for the Sunni tribes has been a key element in the Obama administration’s strategy to contain ISIS, the outcome in Ramadi intensifies doubts about the American position. Sunni tribal leaders told International Business Times weapons the United States and Baghdad promised never showed up on the battlefield, leaving them to make do with makeshift anti-tank missiles, underground tunnel explosives and other stolen weapons manned by untrained volunteer fighters.

The U.S. has independently provided millions of dollars to Iraq as part of a train-and-equip fund, included under the defense budget's Overseas Contingency Operations allotment. That fund allocates money to the Iraqi military, the Kurdish militias, and to training and equipping tribal forces. As part of the fiscal year 2015 budget, the U.S. allocated more than $24 million of the fund to tribal security forces.

American officials said the Iraqi military could still succeed in Ramadi.

"It is possible to see the kind of attack we have in Ramadi, but I am absolutely confident in the days ahead that will be reversed," said Secretary of State John Kerry, speaking  in Seoul Monday.

For ISIS, the capture of Ramadi added a highly strategic target to its realm. Aref Mukhbar Saiad, deputy governor of Anbar province, last month told IBTimes if ISIS were to take the city, it would gain control of “one of the most prominent and commercial transport routes for delivering oil to neighboring countries. ISIS trade would flourish."

Fearing a complete takeover of Anbar province by ISIS, Sunni tribal fighters in Ramadi accepted help from Shiite paramilitary forces after the Sunni militant group took over the city Sunday. The Shiite forces were deploying to the city Monday to try and regain key outposts, local media outlets reported. The loss of the key Anbar province city comes after weeks of fighting and disagreements between the region’s tribal fighters and the Iranian-backed Shiite forces.

A spokesman for the Shiite paramilitary group, known as Hashd al-Shaabi, told Reuters Monday the group received orders to deploy to Ramadi.

"Now that the Hashd has received the order to march forward, they will definitely take part," said Ali al-Sarai, a member of the Hashid al-Shaabi's media wing in an interview with Reuters. "They were waiting for this order and now they have it."

The Sunni tribal fighters in Ramadi previously refused to allow Shiite paramilitary groups to join the fight in the city, citing differences in battlefield strategy and long-standing distrust, tribal leaders said in interviews. The Sunni tribal fighters have fought ISIS almost entirely on their own for four months. Many of the Sunni soldiers were veterans who fought during the “Sunni Awakening” in 2007, when the U.S. recruited Sunnis in local militias to fight al Qaeda.

Other Sunni fighters were part of the Anbar police force, only some of whom have battle experience, Sunni tribesmen fighting with the new brigades said in interviews. The men completed a monthlong training course at a military academy in Anbar and were equipped with weapons stolen from ISIS, or in some cases provided by the Baghdad government.

Despite the training and the build-up of Sunni volunteer forces in Anbar, and more specifically in Ramadi,  relentless onslaughts by ISIS forced the tribal fighters to retreat, allowing the Sunni militant group to take over military outposts.

Although the Shiite paramilitary forces are entering Ramadi to help retake the city, the Sunni tribesmen there told IBTimes they are concerned many of the new fighters are untrained and unskilled.

In Tikrit, the men of Hashd al-Shabi at the Speicher military base, one of the main military outposts in the city, told IBTimes they had not fought before and had come to Tikrit for the money.

The men behind the wall at the military base lobbed mortars and shot artillery in what seemed like an aimless direction. For hours, the men in the militias sat in armored vehicles, their legs hanging out as they rested. Others stood at the front of the base firing randomly.

It is still unclear which groups within Hashd al-Shabi are in Ramadi helping the Sunni tribesmen. One of the largest groups in the Hashd al-Shabi umbrella is the Badr brigade, the Iranian-backed militia. The group was one of the strongest on the Tikrit battlefield, but their spokesmen have yet to say if they are going to deploy to Ramadi.

Iraqi state TV described tanks and other military vehicles entering al-Habbaniyah military camp near Ramadi early Monday. ISIS militants were advancing toward the base to face the Shiite militias.