For the first time since the Islamic State group formally announced its desire to conquer Saudi Arabia last month, government officials said terrorist elements had launched an attack on the Saudi border with Iraq. Four militants, one wearing a suicide vest, attacked a border fortification Monday, killing three Saudi border guards.

The militants' official media wing for Iraq’s Anbar province published a photo essay taking responsibility for the attack, which is the closest ISIS has come to breaching the coveted Saudi Arabian border since it declared the existence of a caliphate in June. The suicide attack signals a change in the group’s strategy to conquer the kingdom. It was both the first direct attack on Saudi armed forces and the first documented attempt by the militant group to infiltrate the country. Despite the government's past efforts to quell extremism, Saudi Arabia has become increasing vulnerable to ISIS advances.

Speaking to Reuters, Iraqi security analyst Mustafa Alani described the border attack as “the first attack by Islamic State itself against Saudi Arabia and is a clear message after Saudi Arabia entered the international coalition against it.”

Saudi Arabia is an active member of the U.S.-led anti-ISIS coalition and a longtime U.S. ally. The kingdom is also the largest producer and exporter of oil and the region’s Sunni powerhouse, making it a triple prize for ISIS and an essential part of its plans to expand the so-called caliphate. The government is well aware of the country's susceptibility to militant infiltration and has made various attempts to quell the rising extremism, that varies from a fence on the border to warming diplomatic ties with Iraq’s Shiite-dominated government.

Saudi Arabia ramped up security at the border in June and more recently built a nearly 600-mile (1,000 kilometer) fence on the border with Iraq to keep out militants. Monday’s attack hit the northern border patrol at al-Suwaif near the city of Arar, the Guardian reported. Arar is more than 700 miles (1,200 km) from Riyadh but territory between the two is not densely populated, and militants may face little opposition if they are able to cross the border and advance toward the capital through open desert.

To boost Saudi presence in neighboring Iraq, officials began talks Sunday on reopening the Saudi embassy in Baghdad, which has been closed since 1990, and a consulate in Erbil, the capital of Iraq’s Kurdistan region. The diplomatic seats are to open "at the earliest opportunity,” the Saudi Press Agency reported.

However, the militant group’s ideology already has succeeded in infiltrating Saudi Arabia. Last month, ISIS claimed responsibility for killing a Danish national in the country and, in a separate incident, security forces arrested around 100 suspected ISIS militants accused of carrying out a November attack on the Saudi Shiite community in al-Ahsa. These attacks were likely carried out by ISIS sympathizers already in Saudi Arabia, who were heeding ISIS leader Abu Bakr al-Baghdadi’s call to attack Saudi citizens, "embitter their lives and make them occupied with themselves instead of us."

Since Baghdadi declared the kingdom to be a new ISIS wilayat (province), the group’s strategy has entailed boosting capabilities of those sympathizers and encouraging sporadic, lone-wolf attacks to shift the focus Saudi’s security forces’ focus away from battling militants in Iraq, Harleen Gambhir, an analyst at the Institute for the Study of War, recently told International Business Times.

Whether the recent attack was the result of direct orders from Baghdadi or of a lone-wolf group of ISIS sympathizers, the threat to Saudi Arabia remains strong. Following the attack, the Saudi Interior Ministry released a statement reiterating the kingdom’s security forces were “determined to thwart … plots to undermine the security and stability of the homeland.”