As the U.S. presses Afghan President Hamid Karzai to sign a long-negotiated security pact, NATO forces say they are investigating civilian casualties from a drone strike in southern Afghanistan.

The strike was "conducted on a known insurgent riding a motorbike" in southern Afghanistan's Helmand province yesterday, NATO's International Security Assistance Force (ISAF) said in a statement released Friday.

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"We are aware that according to the Governor of Helmand Province that in addition to the insurgent being killed, there was one child also killed and two women injured," ISAF said, promising a timely investigation coordinated with Afghan officials. "ISAF deeply regrets any civilian casualties caused by this airstrike yesterday. ISAF is committed to ensuring that all measures are taken to prevent civilian casualties. … We extend our deepest sympathies to the families and loved ones of those killed or wounded.

ISAF Commander Gen. Joseph Dunford, Jr., spoke with Karzai to deliver the same message.

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"He talked to President Karzai directly, expressed deep regrets for the incident and any civilian casualties, and promised to convene an immediate joint investigation to determine all the facts of what happened," a coalition spokesman told ABC.

Karzai has criticized drone strikes and the civilian deaths they have caused, and tensions have risen with the U.S. in the last week, as Karzai refused to sign the Bilateral Security Agreement (BSA) the U.S. has sought. The agreement would allow some U.S. troops to remain in Afghanistan after next year.

On Sunday, a traditional gathering known as the loya jirga approved the BSA and urged Karzai to sign it, but the Afghan president has resisted, demanding that no Americans enter Afghan homes and that the U.S. guarantee peace in Afghanistan. Karzai has suggested he will wait until his presidential term ends in April-a delay U.S. officials at the White House and State Dept. have deemed unworkable.

National Security Advisor Susan Rice visited Afghanistan on Monday to press Karzai to sign the deal, warning that without a "prompt" approval of the BSA, the U.S. will withdraw all troops from Afghanistan.

ABC's Muhammad Lila contributed to this post.