What hardened some diplomats’ conviction that Mr. Karzai wasn’t interested in easing tensions with the U.S. and signing the agreement was his reaction to Friday’s Taliban massacre of international representatives in a Kabul restaurant.

After cursorily expressing his sympathy for the victims, Mr. Karzai used his statement to criticize the U.S. and allied conduct of the war.

“He lost a lot of respect after that,” a senior diplomat said.

Mr. Karzai drew further attention to his discomfort with the U.S. relationship by ordering an investigation into recent Afghan television commercials backing the proposed security arrangement.