As part of the accelerating, high-stakes diplomacy, Reuters has learned, the United States is considering the transfer of an unspecified number of Taleban prisoners from the Guantanamo Bay military prison into Afghan government custody.

It has asked representatives of the Taleban to match that confidence-building measure with some of their own. Those could include a denunciation of international terrorism and a public willingness to enter formal political talks with the government headed by Afghan President Hamid Karzai.

The officials acknowledged that the Afghanistan diplomacy, which has reached a delicate stage in recent weeks, remains a long shot. Among the complications: US troops are drawing down and will be mostly gone by the end of 2014, potentially reducing the incentive for the Taleban to negotiate. Still, the senior officials suggested it has been a much larger piece of President Barack Obama’s Afghanistan policy than is publicly known.

US officials have held about half a dozen meetings with their insurgent contacts, mostly in Germany and Doha with representatives of Mulla Omar, the officials said.

The stakes in the diplomatic effort could not be higher. Failure would likely condemn Afghanistan to continued conflict, perhaps even civil war, after NATO troops finish turning security over to Karzai’s weak government by the end of 2014.

Success would mean a political end to the war and the possibility that parts of the Taleban -— some hard-liners seem likely to reject the talks - could be reconciled.

The effort is now at a pivot point.

“We imagine that we’re on the edge of passing into the next phase. Which is actually deciding that we’ve got a viable channel and being in a position to deliver” on mutual confidence-building measures, said a senior US official. While some US-Taleban contacts have been previously reported, the extent of the underlying diplomacy and the possible prisoner transfer have not been made public until now.

There are slightly fewer than 20 Afghan citizens at Guantanamo, according to various accounts. It is not known which ones might be transferred, nor what assurances the White House has that the Karzai government would keep them in its custody. Guantanamo detainees have been released to foreign governments — and sometimes set free by them — before. But the transfer as part of a diplomatic negotiation appears unprecedented.

The reconciliation effort, which has already faced setbacks including a supposed Taleban envoy who turned out to be an imposter, faces hurdles on multiple fronts, the US officials acknowledged. They include splits within the Taleban; suspicion from Karzai and his advisers; and Pakistan’s insistence on playing a major, even dominating, role in Afghanistan’s future.

Obama will likely face criticism, including from Republican presidential candidates, for dealing with an insurgent group that has killed US soldiers and advocates a strict Islamic form of government.

But US officials say that the Afghan war, like others before it, will ultimately end in a negotiated settlement.

“The challenges are enormous,” a second senior US official acknowledged. “But if you’re where we are ... you can’t not try. You have to find out what’s out there.”

If the effort advances, one of the next steps would be more public, unequivocal US support for establishing a Taleban office outside of Afghanistan.

US officials said they have told the Taleban they must not use that office for fundraising, propaganda or constructing a shadow government, but only to facilitate future negotiations that could eventually set the stage for the Taleban to reenter Afghan governance.

On Sunday, a senior member of Afghanistan’s High Peace Council said the Taleban had indicated it was willing to open an office in an Islamic country.

But underscoring the fragile nature of the multi-sided diplomacy, Karzai last week announced he was recalling Afghanistan’s ambassador to Qatar, after reports that nation was readying the opening of the Taleban office.

Afghan officials complained they were left out of the loop.

On a possible transfer of Taleban prisoners long held at Guantanamo, US officials stressed the move would be a ‘national decision’ made in consultation with the US Congress.

Obama is expected to soon sign into law the 2011 defense authorization bill, including changes that would broaden the military’s power over terror detainees and require the Pentagon to certify in most cases that certain security conditions will be met before Guantanamo prisoners can be sent home.

Ten years after the repressive Taleban government was toppled, a hoped-for political resolution has become central to US strategy to end a war that has killed nearly 3,000 foreign troops and cost the Pentagon alone $330 billion.

While Obama’s decision to deploy an extra 30,000 troops in 2009-10 helped push the Taleban out of much of its southern heartland, the war is far from over. Militants remain able to slip in and out of lawless areas of Pakistan, where the Taleban’s senior leadership is located.

Bold attacks from the Taleban-affiliated Haqqani network have undermined the narrative of improving security and raised questions about how well an inexperienced Afghan military will be able to cope when foreign troops go home.

In that uncertain context, officials say that initial contacts with insurgent representatives since US Secretary of State Hillary Clinton publicly embraced a diplomatic strategy in a Feb. 18, 2011, speech have centered on establishing whether the Taleban was open to reconciliation, despite its pledge to continue its  'jihad’ against NATO and US soldiers.

“The question has been to the Taleban, ‘You have got a choice to make. Life’s moving on,” the second US official said. “There’s a substantial military campaign out there that will continue to do you substantial damage ... Are you prepared to go forward with some kind of reconciliation process?“

US officials have met with Tayeb Agha, who was a secretary to Mulla Omar, and they have held one meeting arranged by Pakistan with Ibrahim Haqqani, a brother of the Haqqani network’s founder. They have not shut the door to further meetings with the Haqqani group, which is blamed for a brazen attack this fall on the US embassy in Kabul and which US officials link closely to Pakistan’s intelligence agency.

US officials say they have kept Karzai informed of the process and have met with him before and after each encounter, but they declined to confirm whether representatives of his government are present at those meetings.

Officials now see themselves on the verge of reaching a second phase in the peace process that, if successful, would clinch the confidence-building measures and allow them to move to a third stage in which the Afghan government and the Taleban would sit down in talks facilitated by the United States.

“That’s why it’s especially delicate — because if we don’t deliver the second phase, we don’t get to the pay-dirt,” the first senior US official said.