Riyadh, Reuters—A suicide bomb and gun attack killed two Saudi border guards on the frontier with Iraq early on Monday, the interior ministry said, in what one security analyst called Islamic State of Iraq and Syria’s (ISIS) first assault on the Kingdom.

Saudi Arabia’s border with Iraq, defended by earth barriers and fences and monitored by camera and radar, has been attacked in the past by mortar bombs fired from a distance, but more targeted strikes are rare.

No group immediately claimed responsibility for the assault, which hit a remote desert area next to Iraq’s Anbar province where both the ISIS militant group and Shi’ite Muslim militias close to Iran are currently based.

The use of a suicide bomber pointed to ISIS, said Mustafa Alani, an Iraqi security analyst with close ties to Saudi Arabia’s interior ministry.

“It is the first attack by ISIS itself against Saudi Arabia and is a clear message after Saudi Arabia entered the international coalition against it,” he said.

Saudi forces have joined US-led airstrikes against ISIS positions in Syria. The Sunni Islamist movement, which has declared its own caliphate and wants to redraw the map of the Middle East, has called for “lone-wolf” attacks against Saudi security forces, the Shi’ite Muslim minority, and foreigners.

ISIS regards Saudi Arabia, which sees itself as a champion of Sunni Islam, as betraying Muslims through its ties to the West. Riyadh has mobilized conservative Sunni clergy to describe the ideology of the Al-Qaeda offshoot as “deviant.”

Monday’s attackers, described by the ministry only as “terrorist elements,” shot at a border patrol near Arar and when security officers responded, one of the attackers was captured and detonated an explosives belt, the ministry statement on state media said.

One of those killed was a senior officer, ministry spokesman Maj. Gen. Mansour Al-Turki told Reuters. Local media, including Al-Arabiya television, named the dead officer as Gen. Oudah Al-Belawi, the head of a border sector. A third officer was wounded, the ministry said

The attacker who detonated the explosives was also killed, as was another attacker who was shot during an initial exchange of fire at 4:30 am near the Suweif border post in Northern Borders province, the statement said.

Saudi Arabia boosted its security on the frontier in July, adding thousands of troops to back up a border guards force, after ISIS seized swaths of territory in Iraq including in Anbar province.

The border remained a formidable barrier for militants, despite the early morning raid, said Alani.

“How many times can you do this attack? The border itself is still very strong. The only fear of them getting into Saudi Arabia is via one of the Kingdom’s other borders,” he said.

The Suweif border post, which Reuters visited in July and is only open to traffic during the annual hajj pilgrimage, is 25 miles (40 kilometers) from the Saudi city of Arar and 50 miles (80 kilometers) from the Iraqi city of Al-Nukhayb, which is held by the government.

Riyadh said last month that the killing of eight Saudis in the Eastern Province was carried out by gunmen linked to ISIS, and that supporters of the group were also behind the shooting of a Danish citizen in November who survived.

In July, an Al-Qaeda raiding party from Yemen killed four border guards on the Kingdom’s southern frontier in a gunfight before two militants entered a police building in the nearby town of Al-Sharurah and exploded a bomb, killing themselves.