The green light from Karzai was in doubt since, while speaking to more than 2,000 elders at the traditional Loya Jirga Sunday, he appeared to hold firm to his earlier stand that he would not sign the document.
In his opening address to the Loya Jirga Thursday, he had said the document should be signed only in April when the country would have a new president following elections.
On Sunday, The New York Times quoted Karzai as telling the meeting he would only sign after further negotiations.
Karzai comments came even though the Loya Jirga had approved the strenuously negotiated security agreement. The agreement will determine how many American forces would remain in Afghanistan and under what terms after the U.S.- and NATO-led coalition forces end their combat operations of the past 12 years and return home by the end of 2014.
The critical approval of the Loya Jirga, though not binding, was needed to move the agreement forward for similar approval by the Afghan Parliament and for signature by the country's president.
In his speech before the Loya Jirga wound up its proceedings, Karzai called for more assurances from the United States about not interfering in the April elections and asked the Obama administration to assure no U.S. soldier will ever again enter an Afghan citizen's home during a military operation, The Washington Post reported.
"Peace, security and a transparent election are preconditions for signing," Karzai declared. "From now onward, Americans don't have the right to raid our homes. If they raid our homes one more time, there will be no" agreement.
The United States all along has been stressing that the agreement should be signed at the latest by the end of this year so it will have adequate time to plan a potential military mission with its NATO allies.
Secretary of State John Kerry, who had worked on the deal until the very last moment, said in a statement Sunday that the Loya Jirga had "powerfully backed" the agreement and that significantly, the Jirga also urged that agreement should be signed before the end of the year.
"I can't imagine a more compelling affirmation from the Afghan people themselves of their commitment to a long term partnership with the United States and our international partners," Kerry said.
"Afghans are rightly taking the lead in providing for their own peace and security. We remain committed to supporting those efforts, and look forward to signing an agreement that will enable us to do so," he said.
Earlier, White House press secretary Jay Carney had said the agreement must be signed before year's end.
"We've made clear that it's imperative that we do it as soon as possible, and further delay is not practical, nor tenable," Carney said, adding failure to meet the deadline "would make it impossible for the United States and our allies to plan for a presence post-2014."
Carney said the United States has negotiated in good faith and provided the assurances Karzai had sought.
Not ruling out the possibility of bringing home all U.S. forces by the end of 2014, Carney had said the BSA does not specify troop presence or duration of any presence of U.S. or NATO troops. That decision will be made by President Obama.
"But we have said that a BSA must be in place because we can't possibly have a further U.S. military presence there without an agreement in place," Carney said.
With the security agreement in place, most estimates are that there may be up to 10,000 U.S. troops beyond 2014. The agreement would also mean billions of dollars of international aid, which Afghanistan sorely needs.
"President Karzai should promise us, he should sign the [agreement] as soon as possible," the Post quoted Sibghatullah Mojaddedi, who headed the Loya Jirga, as saying. "This agreement will be beneficial for the people."
Robert Hilton, spokesman for the U.S. Embassy in Kabul, said Karzai's Sunday speech was being studied.
The Post said the Obama administration has said the BSA draft before the Loya Jirga was the final one, leaving no room for more negotiations.
"From this moment on, America's searching of houses, blocking of roads and streets, military operations are over, and our people are free in their country," The New York Times quoted Karzai as telling the meeting.
Western diplomats told the Times that while Karzai may be indulging in a risky game of brinkmanship, he could still change his mind and sign the document respecting the wishes of the Jirga.