KABUL, Afghanistan — President Hamid Karzai lashed out at his American allies again on Thursday after word came that at least one, and possibly two, NATO drone strikes had killed civilians in southern Afghanistan.

The attacks came at a delicate moment, when talks between Mr. Karzai and the United States over a long-term security agreement had reached an impasse. The Americans have told Mr. Karzai that unless he signs the agreement promptly, they will begin planning for a total withdrawal of American and NATO forces after the end of next year.

Mr. Karzai vowed this week, at the conclusion of a loya jirga, or grand council, that he would cancel the security agreement completely if there was even one more raid that killed civilians.

On Thursday, he said in effect that that moment had come. “For as long as such arbitrary acts and oppression of foreign forces continue, the security agreement with the United States will not be signed,” he said.