The Syrian army broke a three-year siege by the Islamic State on an enclave of the eastern Syrian city of Deir al-Zour on Tuesday, offering a fresh boost to the fortunes of President Bashar al-Assad and his once-flagging army.

After weeks of fierce fighting along the desert roads stretching east toward the city, Syrian soldiers trundled into the besieged garrison of soldiers at a base known as Brigade 137 early Tuesday and then moved on to a cluster of nearby neighborhoods, where they were greeted by wildly cheering residents.

On a day when the Syrian soccer team kept alive the country's hopes of competing for the first time in the World Cup, a mood of national celebration swept ­government-controlled areas of Syria. State television broadcast scenes of ecstatic crowds dancing in the streets and waving Syrian flags in what turned into the biggest national celebration the country has seen since the war erupted six years ago.

The victory also set the stage for a global race to control the rest of the desert province, also named Deir al-Zour, which the United States has also been preparing to liberate from Islamic State fighters still entrenched there.

The relief of the garrison was announced in a Syrian army statement and coincided with a renewed focus by Assad's government on the Islamic State-controlled areas of eastern Syria. It bolsters the argument by Assad that his forces, and not the U.S.-backed fighters farther north, should take responsibility for liberating the remaining areas of Syria controlled by the Islamic State, also known as ISIS and Daesh.

"This is a strategic turning point in the war on terror," said the statement, which was read by a general live on Syrian state television. "It shows the world that the Syrian Arab Army and its allies are capable of destroying the last of the strongholds of Daesh and ending all conspiracies to divide the country."

The push by Assad's forces to relieve Deir al-Zour, in an offensive that began this year, was spurred in part by Syrian concerns about statements from the Trump administration that the U.S. military would soon turn its attention to areas of Deir al-Zour province, analysts say. U.S. officials have said that after securing the city of Raqqa — in the province of Raqqa — where an offensive by U.S.-backed Kurdish and Arab forces is entering its fourth month, they will prepare forces to advance south into Deir al-Zour, in part to prevent further expansion by the Iranian-backed militias fighting alongside the Syrian army in the strategically vital area adjoining Iraq.

The biggest question now is where Syrian government forces will head next, and whether they plan to press on into the rest of the city of Deir al-Zour or turn their attention farther east and south, to the other parts of the province for which the United States is preparing forces.

They are most likely to choose to preempt any further U.S.- backed advances by continuing to head east toward the Iraqi border, and to focus on securing main roads in and out of the country to assert Syria's sovereignty over its borders, said Kamal Alam, an analyst with the London-based Royal United Services Institute.

"This is a very significant victory," he said. "It's the heart of eastern Syria, the key crossroads to Iraq, and it really helps the confidence of the country."

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The army was aided in the fight, as in most of its previous battles, by Iranian-backed militias, as well as by Russian advisers and Russian airstrikes. On Tuesday, Russia's Defense Ministry said that one of its warships in the eastern Mediterranean had fired cruise missiles into the area in support of the Syrian army.

"It is doubtful that regime forces alone could have accomplished this feat," said Tobias Schneider, a London-based analyst who focuses on Syria.

The battle nonetheless showcased the recent improvements in the capabilities of the army, which had been worn down by defeats and defections earlier in the war and had to be rescued by a Russian military intervention in 2015.

Whereas Iranian assistance to Assad has focused on building up militias drawn mostly from Iraq, Russia has focused its efforts on rebuilding the army, Alam said.

"The goal of the Russians was to bring the army back to prewar capacity, and prewar numbers," he said. "Every month, its capability has been improving, and as they freed up territory, they freed up more people to fight."

The victory added to a string of military and political successes in recent months for Assad, who is seeking to consolidate his hold in Damascus after defeats inflicted on the rebels elsewhere in the country laid to rest any hopes they had of ousting him from power.

The relative success of a Russian cease-fire initiative creating de-escalation zones around rebel-controlled areas has also helped free up government forces to focus on the Islamic State-held areas in the country's east.

In recent weeks, loyalist forces have made brisk progress through Islamic State lines across the near-empty desert terrain stretching east from the central city of Palmyra toward Deir al-Zour.

The Brigade 137 base and its surrounding neighborhoods had been under siege since Islamic State fighters overran the city in 2014 and were sustained only by deliveries of food flown to the adjoining airport.

As the advancing forces drove through the vast base, a few dozen of the liberated soldiers ran through the desert and embraced them to cries of "God is great" and "God, Syria, Bashar," according to a live broadcast by state television.

When the relieving Syrian troops reached the adjoining neighborhoods, they were greeted by wildly cheering crowds waving Syrian flags and photographs of Assad. "Our blood, our souls, we sacrifice ourselves for you, Assad," the crowds chanted.

Deir al-Zour is a majority-Sunni city, and the areas that were freed Tuesday have remained loyal to Assad throughout the six-year war, a reminder that by no means all of the country's Sunnis have supported a rebellion largely comprising the Sunni majority.

The army's success there will also give a boost to the Syrian government's intensive efforts in recent months to recruit the support of tribes in the area, reinforcing its bid to wrest back the remainder of the province, Schneider said.

Heba Habib in Stockholm contributed to this report.

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