By Angus McDowall
SHARURA, Saudi Arabia (Reuters) - As Saudi Arabian air strikes have hammered Yemen over the past month, Riyadh's Western allies are believed to have grown increasingly worried by al Qaeda in the Arabian Peninsula (AQAP) taking advantage of the chaos to capture more ground.
Although Riyadh has announced the end of its main air campaign, strikes against the Houthi militia persist, as does the fighting in central and southern areas of Yemen that has distracted its fragmented army, allowing AQAP to benefit.
The Saudis are aware that their bombing campaign has presented opportunities to AQAP, but they see that as a temporary side effect of a strategy ultimately aimed at quashing the militant group by restoring strong government in Sanaa.
"The calculation was that as long as this chaos in Yemen continues, al Qaeda will take advantage, which has a huge impact on Saudi security. It's a major reason they acted in Yemen and convinced the United States to join them," said Mustafa Alani, a security analyst with close ties to Riyadh's Interior Ministry.
Last week a group of tribes and Sunni clerics, including some who have previously been associated with AQAP, moved to take control of large parts of Hadramawt province, including an airport and oil facility, as government forces left.
It set up checkpoints in the region and negotiated with AQAP gunmen in the port of Mukalla, the capital of Hadramawt province, where residents say the militant group has grown increasingly brazen in recent weeks and staged a prison break.
"We keep al Qaeda's presence there in mind and try to get new information. But it doesn't mean we put more soldiers on the border. It just makes us more alert," said Colonel Mansour Ali Qahtani, head of the Saudi Border Guards in Sharura sector.
Unlike along the more populated western stretches of the border, controlled on the Yemeni side by the Iranian-allied Houthis, the Sharura border guards face a desert wilderness where al Qaeda roams free.
There has been no military activity on this stretch of the border since the conflict began on March 26, and Reuters saw no trace of an army presence during the 40-minute drive from Sharura to the al-Wadia border crossing.
AQAP had been well established long before the strikes began, but efforts to curb its presence have been hindered by army infighting and political incoherence. A year ago it staged a cross-border raid into Sharura, killing four security men before the eight militants themselves died in a shootout.
The pretty mud towns and villages of Hadramawt, a wide green canyon that cuts through eastern Yemen's desert plateau, have been plagued by incessant militant violence since Arab Spring protests in 2011 undermined the government in Sanaa.
From the single daily flight to Sharura from Riyadh, long parallel ridges of dunes are visible flowing across the landscape like an ocean swell, separated by wide, shallow troughs of soft, peach-colored sand.
In Yemen this expanse of desert, the Arabian Peninsula's fabled Empty Quarter, is devoid of government control, making it open country for AQAP.
Local border guard commanders play down the dangers posed by AQAP, dismissing last year's raid as an isolated incident and pointing to their extensive defensive system of barbed wire fences and earth berms along the nearby section of border.
The car driver for that AQAP raiding party was a Sharura native, a local man told Reuters. It showed how militancy is as much a domestic problem for Saudi Arabia as a matter of border security. Recent domestic attacks have targeted Saudi Shi'ite Muslims, police and Western expatriates.
But, with Saudi Arabia's home-grown jihadists often taking their ideological and strategic cues from AQAP or Islamic State group, quashing those militant movements in countries beyond the kingdom's borders is an important goal for Riyadh.
The Wadia border crossing - like others into Yemen - remains open to legitimate, documented travelers. Sharura townspeople are descendents of Bedouin nomads whose tribes were present in both countries and still intermarry across the border.
As a young man, Ali bin Sayed al-Say'ari, a white bearded elder herding camels from a pick-up truck in the sands near the border, used to often make the 10-day trek with his beasts to Hadramawt in the days of British colonialism.
Today, he often crosses the border, a sign of the close ties between Yemenis and Saudis on the frontier, and of how political problems in one country directly affect security in the other.
"Saudi Arabia's only fully populated border is the one with Yemen. So it is a very problematic border and definitely for Saudi Arabia this is the backyard of their own security," said Mustafa Alani, an Iraqi security analyst with close ties to the kingdom's Interior Ministry.
(Reporting by Angus McDowall; Editing by William Maclean and Giles Elgood)