Deadly missile and drone attacks by Houthi rebels on Saudi Arabia’s capital and oil infrastructure have exposed the limitations of the kingdom’s armed forces and triggered a risky new phase of the war in Yemen for Riyadh.

Saudi Arabia has one of the largest defense budgets in the world, and its armed forces are equipped with advanced fighters and air-defense systems from the U.S. and other suppliers. But the Houthis have proved an especially resilient foe whose plentiful ground forces and indigenous missile and drone production can effectively counter Saudi military strengths, analysts said.

The Saudis rely on fractious militias loyal to the Riyadh-backed Yemeni government to wage the ground fight, while Saudi fighters and attack helicopters provide air support. The Saudi air-defense effort is hamstrung by a shortage of interceptors critical for shooting down incoming Houthi missiles, along with a reluctance of key Saudi allies, including the U.S., to provide direct military assistance.

Iran has helped the Houthis mount sophisticated attacks on Saudi territory and on its Yemeni allies, including by providing assistance targeting drone and missile attacks, as well as training, intelligence and tactical guidance, The Wall Street Journal has reported.

The U.S. has so far confined its assistance to its Saudi ally to intelligence and other support. President Trump on Friday urged Saudi Arabia to accept military help from Israel against the Houthis. Riyadh, which has an extensive behind-the-scenes security relationship with Israel but no formal diplomatic ties, hasn’t publicly responded to his suggestion.

Instead, Riyadh has turned to Turkey, Pakistan and several European countries for help fending off Houthi attacks with air-defense units and other military assistance deployed to the kingdom. On Sunday, Egypt said it was prepared to provide military help, according to a statement by President Abdel Fattah Al Sisi’s spokesman.

A Saudi official, in a statement to The Wall Street Journal, said: “The kingdom of Saudi Arabia will take all necessary measures to ensure that its citizens and residents are safe and its territory is secure.”The Houthis, he added, “represent a global threat and require a global response.”The kingdom is also looking to recruit foreign fighters to help its Yemeni allies. At the same time, it is also open to a diplomatic solution to the conflict, which otherwise looks hard to contain, people familiar with the Saudi strategy have said.

“They definitely want support to help prevent these catastrophic attacks,” said Allison Minor, a senior fellow at the Center for Strategic and International Studies and former U.S. deputy special envoy for Yemen. “Beyond that, they are looking to demonstrate that they are not isolated in this conflict.”Just as the U.S. has struggled to defeat a seemingly overmatched Iran, the Houthis have built a fleet of mobile missile launchers and deeply buried missile-production facilities that it has used to withstand airstrikes from its much larger neighbor.

The Iran-backed Houthis are likely to be able to absorb attacks from the Saudi-led coalition fighting them, while continuing to deliver blows deep in Saudi territory against softly defended targets, analysts said.

Saudi authorities said Sunday that a hostile projectile struck the southern city of Najran, injuring six. The incident also damaged three homes and three vehicles. That attack came a week after a Houthi strike on the Saudi capital’s international airport killed at least 13 people and injured hundreds. It was the single deadliest attack on the U.S. or its Gulf allies in the conflicts related to the Iran war. Another strike on the airport the following day killed another traveler and wounded 11.

Before the recent surge in Houthi missile and drone attacks, Saudi leaders were more concerned about strikes from across the Persian Gulf. Saudi Arabia struggled to cover the vast country against attacks from Iran and the Iraqi militias Tehran backs, shifting air-defense batteries around to try to protect palaces and energy infrastructure, The Wall Street Journal has reported.

With the threat that the Iran conflict could resume after the U.S. midterm elections next month, Saudi leaders are facing the prospect of a multifront conflict, putting them under even more pressure to quickly tamp down the war with the Houthis, analysts said.

“At some point, they are going to have to find an off-ramp, because the Houthis have this resourcefulness and capacity to respond asymmetrically in a way that can do serious damage,” said Michael Ratney, a former U.S. ambassador to Saudi Arabia and a senior adviser at the Center for Strategic and International Studies.

The Houthis survived a campaign of U.S. airstrikes last year that sought to unlock their grip on the narrow Bab al-Mandeb strait at the entrance to the Red Sea. They also outlasted a Saudi-led coalition that fought for seven years to dislodge the group—a U.S.-designated terrorist organization—after it seized Yemen’s capital San’a and a swath of northwestern Yemen in a civil war.

The Saudi-led coalition’s immediate objective is to dislodge the Houthis from coastal areas along the Red Sea that have allowed them to threaten Saudi shipping in the Bab al-Mandeb strait and to prevent the fall of Taiz, the anti-Houthi forces’ last major urban stronghold in southern Yemen.

The Saudi military has pressed its counteroffensive against the Houthis in recent days, launching waves of airstrikes against rebel-held areas in Yemen and stiffening its air defenses in hopes of warding off new cross-border attacks, Saudi officials said.

“The Saudi armed forces possess highly advanced combat systems and a highly capable air-defense system that can intercept threats and thwart targeting attempts to protect the kingdom’s sovereignty, security, resources, holy sites, citizens and residents,” Maj. Gen. Turki Al-Maliki, a spokesman for the Saudi-led coalition, said during a press conference Thursday in Riyadh.

The kingdom shares a rugged, mountainous, 800-mile border with Yemen, giving Houthi mobile launchers ample cover to fire and quickly relocate.

Saudi forces have successfully intercepted more than 90% of the Houthi missiles targeting areas protected by its U.S.-built Patriot antimissile systems and other air-defense systems, according to a senior U.S. official briefed on the Saudi effort. But over half of the Houthi attacks since the war resumed have been against undefended locations, the official said.

The Houthis have also attacked protected areas using drones and low-flying cruise missiles launched in waves along with ballistic missiles to overwhelm Saudi defenses, the official said.

In the days following the airport attacks this month, Saudi officials said air defenses had intercepted a cruise missile and shot down a drone in the country’s Eastern Province as well as a ballistic missile launched toward the city of Khamis Mushait in southwest Saudi Arabia. Saudi forces have withstood such waves of attacks in the earlier phases of the Yemen war and even managed to curtail them over time, analysts said.

But “it’s very hard to defend against all attacks, and that is the case with militaries like Saudi Arabia’s,” said Minor. “Sometimes what gets through can be catastrophic.”Its vital infrastructure, civilian airports, and Saudi Aramco oil facilities are scattered across vast, sparsely populated deserts. Guarding every potential target simultaneously with fixed missile batteries is logistically impossible.

“What it comes down to is they just don’t have enough missiles, and because they have been attacked from Iraq in the north, from Iran in the east and now from the south, they have to divide their air-defense assets,” said David Des Roches, a former Pentagon official for Arabian affairs.