It was the first such attack since Pakistan's parliament demanded two weeks ago that the U.S. put an end to the strikes.
U.S. officials have indicated privately that they have no intention of doing so. The continuing covert CIA drone program in Pakistan could endanger the Obama administration's efforts to get Pakistan to reopen supply lines to NATO troops in Afghanistan.
Pakistan closed its border crossing to NATO supplies last November, in retaliation for U.S. airstrikes that accidentally killed 24 Pakistani soldiers.
In mid-April, Pakistan's parliament approved new guidelines for the country's relationship with the U.S. It was a move that Washington hoped would pave the way for the supply lines to reopen. But that hasn't happened.
One source of conflict is the parliament's demand that the U.S. provide an "unconditional apology" for the deaths of the Pakistani troops. The U.S. has expressed regret, but has declined to apologize.