Islamabad, Pakistan (CNN) -- Two suspected U.S. drone strikes in Pakistan's tribal region killed 21 alleged militants Tuesday, intelligence officials said.

Two Pakistani intelligence officials said one suspected drone fired four missiles, killing 13 people at an alleged militant hideout in the area of Shawal in South Waziristan, one of the seven districts of Pakistan's volatile tribal region bordering Afghanistan.

The second strike occurred hours later on another suspected militant hideout in North Waziristan. Eight people were killed, the officials said.

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

On Monday, a similar attack killed 10 suspected militants in North Waziristan.

The overwhelming majority of drone strikes have targeted areas in North and South Waziristan. Analysts say the areas are havens for militants fueling the insurgencies in Pakistan and Afghanistan.

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.

Journalist Saboor Khattak contributed to this report.