ISLAMABAD, Pakistan - Militants killed two people they claimed were spies for the United States and dumped their bodies with a warning in a Pakistani border region at the center of a campaign of suspected American missile strikes, an official said Saturday.

Police found the bullet-ridden bodies of the two men on Saturday in the North Waziristan tribal region after a tip from residents, police official Gul Marjan said.

"See the fate of this man. He was an American spy," was written on notes pinned to each of the bodies found in the village of Ghulam Khan, Marjan said. The notes said the men were from the neighboring Afghan province of Khost.

The warning was an indication that Taliban and al-Qaida militants are on the lookout for spies in Pakistan's wild border belt as the frequency of suspected American missile strikes on their hide-outs increases.

At least 18 strikes from what are believed to be unmanned U.S. military and CIA aircraft have hit Pakistan's tribal regions since August, more than three times as many as in 2007. The rugged, mountainous region — where the Pakistani government has never had much control — is considered a likely hiding place for Osama bin Laden and his No. 2 Ayman al-Zawahri.

Many of the cross-border attacks have targeted North Waziristan, a base for Afghan and foreign militants involved in the growing insurgency against the U.S.-backed government in Afghanistan.

Militants have executed scores of Afghans and Pakistanis in the region in recent years for alleged spying. It is unclear whether the victims, many of whom have been beheaded, were marking or informing on targets for U.S. missile strikes.

The missile strikes have drawn condemnation from Pakistan's government, which argues that they undermine the country's sovereignty and its own efforts to combat Islamic radicalism.

"The U.S. administration's reluctance to consider the repercussions of such operations is damaging the whole purpose of global efforts to combat terrorism," Pakistani Information Minister Sherry Rehman said after an attack in North Waziristan on Friday killed some 13 people.

However, the Pakistani government has also renewed its commitment to the seven-year-old war on terror, even as it hopes that President-elect Barack Obama will be more receptive to its arguments.

Rehman said in a statement late Friday that Pakistani President Asif Ali Zardari was urging Washington to halt the attacks. It was unclear if Zardari raised the matter in an overnight telephone call with Obama.