Islamabad, Pakistan (CNN) -- Two suspected U.S. drone strikes in Pakistan's tribal region killed 18 alleged militants Monday, intelligence officials told CNN.

It was the latest in a series of aerial assaults targeting insurgents in North Waziristan, one of seven districts in Pakistan's volatile tribal region bordering Afghanistan.

In one attack, a suspected drone fired four missiles on a militants' vehicle in the Mir Ali area of the district, two intelligence officials said. Six people died in the attack.

In the second attack, a suspected drone fired two missiles at a militant hideout in the same district, killing 12, officials said.

Based on a count by the CNN Islamabad bureau, Monday's suspected drone strikes bring the number to 108 this year, compared with 52 in all of 2009.

The intelligence officials asked not to be named because they are not authorized to speak to the media.

The United States does not comment on suspected drone strikes. But it is the only country in the region known to have the ability to launch missiles from drones, which are controlled remotely.

Security analysts have described North Waziristan as a haven for various factions of Afghan and Pakistani Taliban and al Qaeda militants.

From Journalist Nasir Dawar