People gather at a market place destroyed in a Saudi-led air strike in Yemen's capital, Sanaa, on July 20. The coalition has reportedly been training Yemenis to launch ground operations given the ineffectiveness so far of its air war against rebels. (Khaled Abdullah/Reuters)

After four months of setbacks, fighters backed by Saudi Arabia have seized the offensive in Yemen’s war, taking control of a major city and pressing to expand ground operations against rebel forces.

The shift in momentum after the Saudi-led coalition failed to make headway appears to be due to the arrival since mid-July of hundreds of Yemeni fighters who had been secretly trained in Saudi Arabia. The contingent could help turn the tide in the war, which pits Shiite Houthi rebels from the north against largely Sunni forces aligned with exiled President Abed Rabbo Mansour Hadi.

This week, Saudi-backed fighters attacked areas north of the southern port city of Aden in an apparent effort to enlarge recent gains there against the Houthis. Residents said the fighters are preparing to assault the rebel-held al-Anad base, Yemen’s biggest military airfield, 30 miles north of Aden.

If the Saudi-backed militiamen can consolidate control beyond Aden, they could bring the rebels to the negotiating table or at least carve out a foothold in the south for the Saudi-trained force to gather strength for future assaults.

But it is not clear whether the gains against the Houthis will help end the war anytime soon. The Houthis are tough fighters who are poised to put up stiff resistance. The tribes and militias fighting the Houthis alongside the newly trained force do not all share Saudi Arabia’s goals in the conflict. The escalation also risks further roiling an impoverished country whose people are already on the brink of famine, according to U.N. officials and aid agencies.

A boy holds a poster of former Yemeni president Ali Abdullah Saleh at the site of a Saudi-led coalition airstrike in Sanaa on July 20. The coalition is seeking to restore to power Saleh’s successor, exiled President Abed Rabbo Mansour Hadi. (Hani Mohammed/AP)

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A five-day cease-fire that was supposed to begin Sunday to allow the delivery of food, fuel and other desperately needed aid items never took hold, with both sides continuing to mount attacks.

Meanwhile, al-Qaeda-linked extremists appear to be taking advantage of the chaos to boost their presence. Residents in Aden say the extremists are participating in the assaults by the Saudi-led coalition, perhaps seeking to win local support.

Brig. Gen. Ahmed Asseri, a spokesman for the coalition, called the Aden offensive a “positive development” that was “only the beginning” and denied that Yemen’s powerful al-Qaeda affiliate — al-Qaeda in the Arabian Peninsula, or AQAP — was involved.

Speaking by telephone last week, Asseri said Aden is being used as a beachhead for more attacks to drive back the Houthis and restore Hadi’s “legitimate government.” In February, the Houthis ousted Hadi, who now operates from Saudi Arabia, after seizing Yemen’s capital, Sanaa, and then swept south to capture a vast swath of territory, including Aden.

The Saudi-led coalition of mostly Arab countries responded in March by launching its air war. Saudi Arabia views Yemen, a country of 25 million people, as its back yard and the rebels as proxies of its chief enemy, Iran — an assertion the Houthis deny.

Analysts say the plan to use Yemeni ground forces has grown more urgent for the coalition since Iran reached an agreement with world powers this month to limit its nuclear program. Saudi officials, the analysts say, fear that the lifting of sanctions under the accord, if approved, will free up billions of dollars that Iran will use to help allies in Syria’s civil war and the Houthis.

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“The Saudis feel they need to be able to get a foothold back in Yemen again before Iran starts to mobilize and get more committed to this war,” said Christopher Davidson, an expert on Persian Gulf countries at Durham University in Britain.

Rumors of the coalition’s training program have circulated for several months, although few details have been disclosed. Coalition countries have refrained from sending their own armies to fight in Yemen, fearing high casualties in a protracted conflict.

A former Saudi diplomat who is familiar with the program confirmed that several thousand Yemeni nationals are undergoing boot-camp-like training in at least two military facilities in southern Saudi Arabia. The training began as the air war started, he said, speaking on the condition of anonymity because he was not authorized to discuss the issue.

Asseri, the coalition spokesman, also confirmed the existence of the training program but declined to give details.

Mustafa Alani, an expert on Middle East security issues who is close to Saudi officials, said the coalition plans to train as many 10,000 Yemeni troops at the Saudi sites. Recruits include tribal fighters, former military officers and Yemenis living in Saudi Arabia, he said. The first batch of graduates participated in the assault on Aden, which the coalition chose to attack first because of strong local opposition to the Houthis, he said.

“The idea is to replicate what happened in Aden northward, until these forces reach Sanaa,” Alani said. Or, he noted, the force could apply enough pressure on the Houthis to force them to enter peace negotiations.

In mid-July, hundreds of the newly trained fighters entered Aden from coalition warships and linked up with militiamen who had been fighting the Houthis in the city for months, according to fighters and residents. They said the force brought with them armored personnel carriers, which proved decisive in gaining control of the airport, port and most city neighborhoods.

“There are about 600 of these new fighters who joined us,” said Wadhah al-Dubaish, 40, an anti-Houthi fighter in Aden. He described the Saudi-trained force as “organized and well-armed.”

Dozens of military advisers from Saudi Arabia and the United Arab Emirates, a coalition member, are also in Aden to assist the force, according to Dubaish and the former Saudi diplomat.

Despite the coalition’s disavowals, its strategy also appears to be drawing support from radical Islamists. Militants with suspected ties to AQAP are taking part in the anti-Houthi fight in Aden, residents and some fighters in the city said.

The extremist Sunni group has seized territory in the nearby province of Hadramaut in recent months, and analysts warn that it could use the area to stage attacks on the West. During its air war, the Saudi-led coalition is not known to have attacked militants from AQAP, which views the Shiite Houthis as apostates.

“I saw groups of militants in Toyota pickups carrying the al-Qaeda flag” in Aden, said Samir al-Samei, who works for a nongovernmental organization in the city. “They were dressed in black, and their faces were covered.”

The planned escalation is also certain to prolong fighting that since March has left more than 3,500 people dead and 1.3 million displaced.

The Houthis say their forces are regrouping outside Aden. “We will make this a graveyard for the invaders,” said Ahmed Ghuraib, the Houthi-appointed governor of Lahij province, north of Aden.

There are other signs of trouble, such as anti-Houthi militias with agendas that are at odds with the coalition’s objectives. Prominent among the militants in Aden are southern separatists who call for a split with the north and oppose Hadi’s exiled government.

“Those forces are not loyal to Hadi, and they are not loyal to Saudi Arabia,” said Katherine Zimmerman, an expert on Yemen at the American Enterprise Institute. But for now, the disparate groups are united against the Houthis, who appear overextended and vulnerable to attack, she said.

Jamal Khashoggi, a prominent Saudi journalist, said officials in Saudi Arabia also believe the Houthis are militarily vulnerable. “There is feeling now that success will breed more success,” he said.

But, Khashoggi warned, a major escalation would “be a painful battle for all Yemenis.”

Ali al-Mujahed in Sanaa, Yemen, contributed to this report.

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