From their post on a rocky hilltop, a pair of Saudi border guards man a .50-caliber machine gun and use binoculars to scan the dry scrubland that separates this kingdom from its war-torn neighbor to the south, Yemen.
The scene before them appeared peaceful Friday: The craggy peaks that rise beyond a riverbed were spotted with goats, cows and families of baboons. But later that day, mortar rounds fired into Saudi territory from Yemen killed three soldiers and injured two others stationed along the frontier, state media reported Saturday.
It was the latest in a series of border skirmishes that have killed six of the kingdom's troops since a Saudi-led coalition began airstrikes March 25 against rebels known as Houthis, who have seized large parts of Yemen. The Saudi Defense Ministry said that its forces returned fire, and that 500 Houthi fighters have been killed in the clashes.
"Our border is a red line," said Lt. Col. Hamed Alahmari, a spokesman for the Interior Ministry guards who patrol the highly porous frontier that stretches about 1,000 miles through mountains and desert.
Officials in Saudi Arabia, the region's Sunni Muslim power, say the air campaign is dealing a decisive blow against the Houthis, whom they view as tools of aggression used by Shiite Muslim-led Iran in an expanding proxy war.
Coalition airstrikes have destroyed fighter jets, ballistic missiles, antiaircraft guns and other military hardware held by the Houthis and their allies, who have taken control of large parts of Yemen.
However, residents say the strikes have done little to reverse the territorial gains of the insurgents and restore exiled President Abdu Rabu Mansour Hadi to power in the quickly fragmenting country.
Security experts question whether the coalition can achieve its goals through airstrikes alone. Saudi officials have not ruled out sending in tanks, artillery and other ground forces massed along the frontier. But Saudi leaders appear wary of such a move against the Houthis, hardened guerrillas who belong to an offshoot of Shiite Islam known as Zaidism.
The last time the Saudis fought the Houthis in the rugged mountains of northern Yemen, in 2009, more than 100 of their men were killed. Pakistan's parliament voted Friday to stay out of the conflict, a blow to the Saudis, who had reportedly asked the country to send troops, fighter jets and warships.
"This [war] will turn Yemen into Saudi Arabia's Vietnam," said Mohammed al-Kibsi, a veteran journalist and commentator in Yemen's capital, Sana, where the Houthis seized control in September.
International aid agencies warn of an increasing humanitarian disaster. The fighting has killed at least 643 people, displaced more than 100,000 and laid waste to Aden, the commercial hub where Hadi took refuge before fleeing late last month to Saudi Arabia.
Al Qaeda in the Arabian Peninsula, which is widely viewed as the terrorist network's most dangerous franchise, has capitalized on the chaos to extend its territorial reach and stage a prison break that freed scores of supporters, including a senior militant leader.
"The Saudis understand there is no military solution in Yemen," said Mustafa Alani, a security studies scholar at the Gulf Research Center in Dubai, United Arab Emriates. "There are two objectives of this operation. The first is to destroy the military backbone of the Houthis. … This they are doing very well. Second, to weaken the Houthis to the point that they go back to the negotiating table."
Saudi Arabia and its allies have become increasingly concerned by what they view as an aggressive campaign by Iran to project its influence across the region, which they do not believe the United States is taking seriously enough. The Obama administration's pursuit of a nuclear deal with Iran has added to the growing mistrust of U.S. intentions.
The Islamic Republic is a major sponsor of the governments in Syria and Iraq and of the Lebanese militant group Hezbollah. Coalition members view the Houthi uprising as another attempt by Tehran to put its clients in charge of Arab capitals, this time in Saudi Arabia's backyard.
Brig. Gen. Ahmed Asiri, a spokesman for the Saudi-led campaign, accuses Iran of providing large quantities of weapons and training to the Houthis, a charge that Tehran has denied.
"Once they got control of Sana, they signed a contract with an Iranian airline, 14 flights a week," he told The Times. "To do what? We did not know that there is tourism coming from Yemen to Iran or from Iran to Yemen.
"We accept that our neighbor has a very strong army," he continued, "but not the militias."
Pro-Hadi fighters claim to have captured two Iranian military officers who were advising the insurgents during fighting Friday in Aden. Reports from Yemen said the officers were members of an elite unit of Iran's Revolutionary Guard, charges Tehran also rejected.
Leaders in Iran have denounced the Saudi-led military campaign with increasing vitriol. In a speech Thursday, Iran's supreme leader, Ayatollah Ali Khamenei, lashed out at the government formed by Saudi Arabia's newly installed King Salman, which he characterized as "inexperienced youngsters" who had replaced their predecessors' restraint with "barbarism."
U.S. officials believe that Iran is providing some military aid to the Houthis, and have increased the logistical support, intelligence and weapons they are contributing to the Saudi-led campaign. But they do not believe that Iran is directing the Yemeni militia.
Far more important to the Houthi campaign, they say, are the bases, military equipment and fighting power provided by elements of the Yemeni armed forces who are still loyal to Ali Abdullah Saleh, the former strongman deposed in 2012.
Saddam Abu Asem, a commentator whose columns have appeared in a number of Yemeni news outlets, said the coalition campaign is winning support for the Houthis among residents who no longer consider Hadi a legitimate president because he invited airstrikes that are killing the country's people and destroying its infrastructure.
But he said the relentless bombardments and a blockade on Yemen's air and sea ports have weakened the pro-Houthi forces, who also face growing domestic pressure over soaring food prices, fuel shortages and power cuts.
"There is talk that these parties are looking for a way out of the crisis," Abu Asem said.
Naif Qanes, a member of the "revolutionary committee" tasked by the Houthis to run the government, said the insurgents are ready to take part in any talks that could lead to a solution and "stop these barbaric assaults against Yemen."
Houthi leaders say such talks cannot take place in any of the countries involved in the bombings. Analysts have suggested that Oman, the only member of the six-nation Gulf Cooperation Council that is not taking part in the airstrikes, could provide a neutral venue.
The Saudis and their allies insist that the Houthis recognize Hadi's legitimacy and surrender the arms they have taken from the state. But they recognize that Hadi has limitations, including a lack of charisma or an effective power base, according to analysts who are close to the region's leaders.
Alani, the security studies scholar, suggested that a figure with broader national appeal might be brought into a transitional government.
"You cannot overlook the question of legitimacy," he said. "But at the same time there is a vacant position of vice president."
Observers in Yemen are more skeptical that Hadi will be able to return to power. Too much blood has been shed, said Abu Asem.
"This is may affect the future of Yemeni unity."
Times staff writer Zavis reported from Jizan and special correspondent Al-Alayaa from Sana. Times staff photographer Carolyn Cole contributed to this report.