By Angus McDowall

JIZAN Saudi Arabia (Reuters) - Gains by the Shi'ite Houthi rebel movement in Yemen are ringing alarm bells in Saudi Arabia, concerned for what it means for its vulnerable southern border, already the conduit for a constant flow of illicit activity.

The Houthis control much of the territory along the 1,700-km (1,060-mile) frontier, which traverses high mountains and vast expanses of dune desert, and five years ago fought a brief border war with the world's top oil exporter.

With no border patrols or guard posts to the south, the only obstacles for smugglers, economic migrants and groups the Saudis worry about even more, such as al Qaeda, are on the Saudi side.

"We are working alone," said Lieutenant Colonel Hamid al-Asmari of the border guards in Jizan province, one of the most active parts of the frontier.

It is only a few hundred meters from the sandbagged emplacement of the Saudi border guards in Jizan to the al-Mashnaq arms market, in a tiny mud village across a broad wadi.

When the guards pause for communal prayers each Friday, they can hear Houthi sermons broadcast from across the wadi. When they peer through binoculars they see the group's slogans daubed in paint on the walls: "Death to America! Death to Israel!"

After the 2009-2010 war, fought in this very district, many villages were evacuated and abandoned, and now lie in a resurgent wilderness where butterflies dance over covered crumbling walls and shell-pocked houses.

Around 200 Saudi soldiers died in the conflict, triggered by a dispute between Riyadh and the Houthis over where the border lay.

The Houthis have had control of large swathes of north Yemen since they built a following among the region's tribes in the early 2000s, campaigning for the rights of Zaydi Shi’ite Muslims.

After fighting six inconclusive wars with the central government, they took control of the capital, Sanaa, in September and are now a major force in Yemeni politics.

The movement rarely speaks to Western media and did not respond to a request for comment on this story.

Sunni Saudi Arabia is alarmed by the Houthis links to Iran, its rival for influence in the Middle East, and fears they may seek to emulate the king-making role played in Lebanon by its Shi’ite militia Hezbollah.

The Saudis are also concerned about another strategic threat emanating from Yemen: it is home to Al Qaeda in the Arabian Peninsula, which has declared war on the kingdom's ruling Al Saud family and in July staged a cross-border raid further east.

For now, enmity between the Houthis and AQAP makes their presence in the Jizan border area improbable. Sunni AQAP has declared the Shi’ite Houthis heretics and staged suicide bombings against them, while the Houthis have pledged to rout the militant group from Yemen.

SMUGGLERS

For the guards patrolling the border, then, the biggest concern is that its frontier lies in the hands of a group whose main constituents are local tribesmen who live off smuggling.

Last year in Jizan province alone, border guards detained 235,000 people trying to cross the border illegally, seized 2,800 weapons including assault rifles, hand grenades and small rockets, and 16 tonnes of hashish, Asmari said.

Saudi Arabia is working on a new border road with a fence running alongside as well as tall posts for cameras and radar equipment that should allow guards to maintain a watch on the entire length of the frontier and dispatch patrols quickly.

But the project will take years to complete. Although it has been in the works for many years, it was slowed down by the difficulty of the terrain, by legal disputes over land ownership and by the war.

In the meantime, this will remain one of the most dangerous places in Saudi Arabia. Last year two border guards were killed in Jizan province by smugglers. Guards in both the observation post and in another, much higher position far into the mountains, said they are occasionally shot at from a distance.

Driving along a dirt border track where scrubby bushes and acacia trees provide extensive ground cover, Asmari and his patrol unit stop to watch a group of men scurrying for cover.

Once they are 50 meters or so into the bushes, more or less back into Yemeni territory, the men turn to wait for the Saudi patrol to leave.

Further on, another group stands in the scrub eyeing the Saudi guards and one briefly raises what looked like a weapon.

The tension is in stark contrast to the district's natural beauty and tranquillity. Wandering herds of camels, goats and cows with fatty shoulder humps daily cross from the Yemeni side to graze along the lush wadis before peaceably ambling home.

(Editing by Sonya Hepinstall)