WASHINGTON (Reuters) - CIA director John Brennan said on Wednesday recent attacks in Saudi Arabia bore the hallmarks of Islamic State, and that the militant group was a very serious threat to the kingdom.

Suicide bombers struck three cities across Saudi Arabia earlier this month, killing at least four security officers in an apparently coordinated campaign of attacks on the penultimate day of the Muslim holy month of Ramadan.

“Those attacks bear the hallmarks of ISIL,” Brennan said at an event hosted by the Brookings Institution think tank, using an acronym for Islamic State.

The explosions struck in Jeddah, Qatif and a security headquarters in the holy city of Medina, an attack Brennan described as “unprecedented”.

The attacks were not claimed by any group although the Saudi government believes Islamic State is responsible after detaining 19 suspects linked to the five attackers.

Brennan said that while al Qaeda still posed a threat to Saudi Arabia, which had launched a fierce crackdown on the militant group in the early 2000s, Islamic State posed a greater danger to the kingdom.

Islamic State militants have carried out similar bombings in the U.S.-allied, Sunni Muslim-ruled kingdom in the past year, targeting minority Shi’ites and Saudi security forces.