Afghan President Hamid Karzai speaks during the opening of the loya jirga in Kabul on Thursday. (OMAR SOBHANI/REUTERS)

Afghan President Hamid Karzai will continue to defy U.S. threats to walk away from a security agreement between the two countries and plans to reiterate in a speech to a grand council Sunday that he will not sign it before spring, his spokesman said Saturday.

“They have waited this long, they can certainly wait five more months,” spokesman Aimal Faizi said Saturday of the Americans. The Obama administration has characterized the deal, whose terms Karzai had agreed to last week, as a “final offer” that must be completed by the end of this year.

Karzai appears certain that the administration is bluffing, saying through the spokesman that he does not believe the United States will resort to the “zero option” of canceling plans to leave a residual troop force here to train the Afghan military and continue counterter­rorism operations after it withdraws its combat forces in December 2014.

But the administration, for its part, has also thought that Karzai would change his tune after opening the assembly of more than 2,500 tribal elders and other leaders, called a loya jirga, on Thursday with a speech vowing to delay finalizing the deal until after Afghanistan’s presidential election in April.

The tension has escalated into a remarkable stare-down between two leaders who say they want the same thing — U.S. troops providing long-term training and support for the Afghan military — but find themselves on the brink of walking away from that partnership.

“The core of the issue,” Faizi said, is a “lack of trust” between the two governments.

As the elaborate game of diplomatic chicken neared a climax, the Obama administration was publicly silent Saturday, while continuing talks in private.

U.S. officials in Washington said they were heartened by statements coming out of the loya jirga, where some members seemed to “understand the importance of signing sooner rather than later,” a senior administration official who was not authorized to discuss the tense situation said, speaking on the condition of anonymity.

The officials pointed in particular to a statement Saturday by Sebghattulah Mojaddidi, the chairman of the loya jirga, who told Afghanistan’s Tolonews that he hopes Karzai will avoid a showdown with the Americans over the timetable, saying it would harm Afghanistan. “If the U.S. has accepted our terms, then we should not delay,” Mojaddidi said.

But some jirga members have spoken out in opposition to particular portions of the deal, including a grant of immunity from Afghan legal prosecution for U.S. troops.

In recent days, Secretary of State John F. Kerry and Defense Secretary Chuck Hagel have signaled that the administration has little patience for a continued delay or for any substantive ­changes in the document. Faizi described a phone conversation Friday between Karzai and Kerry as “friendly but tense” and said that Kerry had threatened a complete withdrawal.

Administration officials consider the signing date to be non­­negotiable, citing the need for at least a year to plan future deployments and to allow coalition partners, including Germany and Italy, to plan for a residual troop presence that they have offered.

Having drawn a line in the sand after a year of negotiations, the administration is also highly reluctant to suffer a repeat of the diplomatic embarrassment that ensued when a similar deal with Iraq went to the wire before ending with a total U.S. military withdrawal at the end of 2011.

While U.S. officials have publicly portrayed the current draft agreement as written in stone, officials said the administration is willing to consider small ­changes based on loya jirga concerns, if that will move the assembly to press Karzai to sign.

Faizi said Saturday that Karzai is also seeking additional “guarantees” that the administration won’t interfere in the coming elections, that it will help stabilize security in Afghanistan and that remaining U.S. forces won’t abuse their power to enter the homes of Afghans.

“There should be flexibility on two sides,” he said.

Meanwhile, outside the confines of the jirga, fresh tension arose between the two governments for a second consecutive day over the deaths of two Afghans last week.

On Friday, Karzai accused U.S. troops of killing the two men, whom he described as civilians, in an operation in the eastern province of Nangahar. Coalition military commanders responded with outrage, saying the men were “armed insurgents.”

“It’s really unfortunate some people are using allegations of civilian casualties for political purposes,” said a senior coalition official who spoke on the condition of anonymity to discuss the matter freely. The official, who noted that the mission in question was led by 100 Afghan soldiers but included 17 coalition advisers, said Karzai’s remarks were “complicating” efforts to maintain U.S. military support for the security agreement.

But Faizi said Saturday that Karzai stands by his initial statement about the incident, adding that the governor and residents of Nangahar are furious. “Bad behavior happened,” Faizi said.

The incident is giving Karzai cover to express other long-standing grievances with the United States, most notably a lingering suspicion that the administration supported his opponent during his 2009 reelection campaign.

Faizi said repeatedly that Karzai wants to postpone a final decision on the security agreement as leverage for ensuring that the United States both stays out of and helps secure the upcoming Afghan elections.

“The United States can make sure there is no interference in the Afghan elections as there was in the past presidential” race, Faizi said.

Meanwhile, the U.S.-led coalition is also facing a headache in neighboring Pakistan over continuing drone strikes on its territory.

Imran Khan, the political leader of Pakistan’s Khyber Pakhtunkhwa province, led a day-long protest near the city of Peshawar against NATO convoys that travel through the region to and from landlocked Afghanistan.

On Sunday, workers affiliated with Khan’s Movement for Justice party plan to escalate the demonstration by placing shipping containers and other barriers on NATO convoy routes. The blockade will last indefinitely, party officials said, despite calls for restraint from Pakistani Prime Minister Nawaz Sharif.

DeYoung reported from Washington. Haq Nawaz Khan in Peshawar, Pakistan, and Mohammad Sharif in Kabul contributed to this report.