Islamabad, Pakistan (CNN) -- Pakistan has issued a strongly-worded statement condemning a deadly, suspected U.S. drone strike in the country's tribal region.
"Drone attacks have become a core irritant in the counter-terror campaign," a statement from the Ministry of Foreign Affairs said Wednesday. "We have repeatedly said that such attacks are counter productive and only contribute to strengthen the hands of the terrorists."
Pakistan's Foreign Secretary has lodged a strong protest with the U.S. Ambassador, according to the statement, and the government has "taken up the matter with the U.S. at all levels."
Six suspected militants were killed Wednesday when a suspected drone fired two missiles at a hideout in South Waziristan, Pakistani intelligence officials told CNN.
The officials asked not to be named because they are not authorized to speak to the media.
Drone strikes are now carried out autonomously by the CIA, a senior Pakistani intelligence official told CNN earlier. "It's unilateral action," he said as he expressed dismay at Wednesday's strike.
The U.S. does not officially acknowledge the Central Intelligence Agency's secret drone program in Pakistan, but it is the only country operating in the region with the capacity to carry out such strikes. It was once widely believed that the program had the tacit approval and cooperation of Pakistan's intelligence agency.
Pakistani intelligence officials say Islamabad has asked the U.S. to curtail its drone strikes in remote tribal areas.
"We are not necessarily saying stop. What we are saying is irrespective of tactical games, this is not useful in the long run," one source told CNN. "We need a relook and to not be too trigger happy and be selective."
Wednesday's strike is the first since March 17, when a drone strike in North Waziristan killed 44 people -- most of them civilians, two Pakistani sources said. It's the 19th strike this year, according to a count by CNN's Islamabad bureau.
Such strikes have inflamed tensions between Pakistan and the United States. Pakistan has formally asked the United States for an apology.
On Monday, Pakistan's intelligence chief, Lt. Gen. Ahmed Shuja Pasha, met with CIA Director Leon Panetta at CIA headquarters to voice his government's complaints about CIA activities in his country.
U.S.-Pakistani relations have been strained for some time because of ongoing American intelligence operations within Pakistan's borders. Tensions came to a head recently when when CIA contractor Raymond Davis was arrested after killing two Pakistani men in Lahore. Although the United States said Davis was a security officer assigned to protect CIA officers, the Pakistanis maintained he was actually a spy whom they had been unaware of prior to the shooting incident.
Pakistani officials insisted this week that the United States give them details about other CIA contractors in the country.