Saudi Arabia and its allies plan an ambitious ground offensive on multiple fronts in Yemen. It may be inevitable if they want to defeat Iranian-backed Shiite rebels but it also carries enormous risks, from the inhospitable, mountainous terrain and a possible guerrilla war to al-Qaida militants waiting in the wings.

The first main objective of a ground assault would be to secure the southern port of Aden and its immediate vicinity to allow the return of Western- and Gulf-backed President Abed Rabbo Mansour Hadi, who fled the Arabian Sea city last month in the face of a push south by the rebels. There, the troops would begin the task of building a new Yemeni army, replacing a military fragmented by the conflict.

The more daunting part of the plan is for Saudi-led forces to cross the border from Saudi Arabia into Yemen, according to Egyptian and Yemeni military and security officials briefed on planning. That means entering a mountainous landscape that is the home stronghold of the Shiite rebels, known as Houthis. There in Saada province, they have battled Yemen's military to a standstill in six wars over the past decade.

The ultimate goal is not to annihilate the Houthis but to push their fighters back to their home provinces of Saada and Omran in the north and force them to negotiate. The officials spoke to The Associated Press on condition of anonymity to discuss the plans.

Officials have said the offensive will only start when airstrikes — a week old on Wednesday — sufficiently weaken the Houthis and their allies, military and police units loyal to Ali Abdullah Saleh, the longtime autocrat who was removed in 2012 and replaced by Hadi. The Houthis swept out of the north last year to capture the capital Sanaa in September in a whirlwind offensive backed by Saleh's troops that swallowed much of northern Yemen.

There are difficulties in the strategy beyond the fighting itself.

Coalition forces — likely to be mainly Egyptian, Saudi and Pakistani troops — are not looking to have to occupy large parts of Yemen. Instead, they will rely on Yemeni allies: Sunni tribesmen, local militias and whatever they can build of a Yemeni military. Some 70 percent of military units are commanded by officers loyal to Saleh, including the best armed and trained ones.

But tribesmen and the militias can be unpredictable allies.

For example, a powerful pro-Hadi militia in the southern Abyan province pulled out of Aden, where they were helping defend the city, because they were worried the Houthis would take advantage of their absence to move on their home province. The militia also complained of what it called lack of organization among Aden fighters.

Saudi Arabia has for decades sought to have leverage in its strategic neighbor by funding Yemeni politicians, tribal leaders, army officers and media organizations. It will be looking to its longtime alliance with Sunni tribesmen to aid the ground offensive.

Some of these Sunni tribes, however, have entered alliances of convenience with militants from al-Qaida's Yemen branch to prevent the Houthis from capturing more territory. That could put the alliance in the awkward position of being essentially in alliance with a terror group whose declared aims include toppling the Saudi royal family. And even if they share an enemy, coalition troops must be wary of the possibility of attacks by al-Qaida and other Islamic militants.

But then, if the Houthis are not reined in, al-Qaida's branch in Yemen — already considered the terror network's most dangerous — will likely only grow stronger as it benefits from the chaos.

Aden must be stabilized before any coalition landing there can take place, mostly likely by Egyptian troops under Saudi air cover.

Fighting in Aden has been fierce the past days. Most of the city is in the hands of militias and army units loyal to Hadi. But Houthis and Saleh's forces hold several locations. The airport has changed hands several times in the past week. Houthi fighters have been hammering the center with rocket-propelled grenades fired from rooftops in the city-suburbs, including from one of Aden's largest shopping malls.

Pro-Hadi militias, known as the Popular Committees, have been stretched thin and likely would have been overrun if not for intense airstrikes the past 48 hours against Houthi and pro-Saleh positions, Yemeni officials said. Drones and manned aircraft fly surveillance missions over the city round the clock. Airstrikes have also prevented Houthi and pro-Saleh forces marching from neighboring areas from reaching Aden to reinforce their side.

But if landing in Aden will not be without perils, crossing the border from Saudi Arabia into Yemen could be treacherous.

To prepare for the mission, the Saudis are currently holding joint war games with several hundred Pakistani troops who are veterans of guerrilla warfare against militant Islamic groups in their country's tribal regions along the Afghan border. The exercise is taking place in southwest Saudi Arabia, where the mountainous terrain mirrors that of the Houthis' Saada stronghold. Contacts are believed to be underway between Saudi Arabia and Pakistan on a final decision on deploying a Pakistani contingent in any assault.

Egyptian advisers have also been stationed near the Saudi border with Yemen, joining the Saudis and other coalition partners in laying out plans for the offensive.

The ground assault in Saada aims to reduce the Houthis' military capabilities, with a side objective of killing or capturing top officials in the movement, including leader Abdul-Malek al-Houthi, the officials said.

Already, the Yemeni officials said, most of the Houthis' bases, arms and ammunition depots and airfields have been destroyed or incapacitated by airstrikes, along with some civilian infrastructure like bridges.

But invading troops will face a mountainous terrain and a hostile population. They will also face rebels fighting on home turf and battle hardened by years of wars. The Saudi military has seen almost no ground combat since the 1991 Gulf War over Kuwait. One exception: a brief crossborder assault into Yemen to fight the Houthis in late 2009-early 2010 that left 130 Saudi troops dead and achieved little.

Egypt has its own bad history in Yemen. In the 1960s, socialist President Gamal Abdel-Nasser sent forces in support of republicans who toppled the monarchy in a 1962 coup. An estimated 10,000 Egyptian troops died in that Yemeni civil war, which ended in 1970.

Egypt and Saudi Arabia at that time were on opposing sides: The kingdom, Jordan and Britain backed the monarchy, which was a dynasty belonging to the same Shiite Zaydi branch that the Houthis follow. Zaydis, who almost exclusively live in Yemen, are thought to make up about 30 percent of Yemen's estimated 25 million population.

In all likelihood, said the security and military officials, the Saudis and their allies would not stay long in Saada, though there is discussion of trying to carve out a border strip of control inside Yemen to prevent incursions.

A possible third target of a ground offensive is to secure a beachhead in the province of Taiz, which overlooks the strategic southern entrance to the Red Sea known as Bab al-Mandab. The Houthis and their allies took over the provincial capital last month, but there has since been a wave of street demonstrations to protest against their presence.

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Hendawi is the AP's Cairo bureau chief and has frequently traveled to Yemen on assignment. AP reporter Ahmed al-Haj in Sanaa, Yemen, contributed to this report.