The 50 committees of the "loya jirga" gathering of about 2,500 chieftains, tribal elders and politicians gave their assessment of the deal one by one at the conclusion of four days of discussions under tight security in Kabul.
Almost all of the first 20 committees to declare endorsed the painstakingly negotiated Bilateral Security Agreement (BSA) governing the presence of US troops in Afghanistan after 2014.
Some even suggested adding an extra US base in the province of Bamiyan, while more than half of them urged Karzai to get the BSA signed into effect before the presidential election next year.
Karzai's spokesman Aimal Faizi told AFP that the president would explain the reasons for his stance in his closing speech to the jirga.
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Karzai's conditions also include an end to military operations on Afghan homes and cooperation in the peace and election processes, Faizi said yesterday.
The State Department warned that failure to sign the pact -- which governs the conditions of any post-war American counter-terrorism and training mission in Afghanistan -- could jeopardise billions in vital aid to the war-torn country.
"Karzai doesn't have the right to say this, he is making a mistake," Sebghatullah Mujadidi, the head of the jirga, said on Saturday.
"They (the Americans) have accepted all the conditions set out by him and us. It would hurt Afghanistan if he does not accept it," he added.
Amir Mohammad Akhnudzada, a delegate from the volatile southern province of Helmand, said: "I think President Karzai should respect the decision of the Afghan elders, and all the delegates want this Bilateral Security Agreement signed as soon as possible."