Mullah Baradar was detained in Karachi in January
Pakistan's main intelligence agency has eased restrictions for US investigators to interrogate a top Afghan Taliban commander, officials have told the BBC.
Security sources say the Americans began getting "limited access" to Mullah Abdul Ghani Baradar last month.
He was caught in late January during a raid on a madrassa near Karachi.
Mullah Baradar's capture came amid a major Nato-led offensive against the Taliban in southern Afghanistan and was hailed as a significant breakthrough.
US media reports suggest the Americans are satisfied with the information they are getting from the detained Taliban leader.
Correspondents say that direct US access to Mullah Baradar was minimal at first.
But since the Inter Services Intelligence (ISI) eased restrictions on American investigators, they have been participating regularly and directly in interrogation sessions for at least a month, US officials say.
"These things take time," one US military official told the Reuters news agency. "It takes time to get the information and it takes time to check out that information."
"He started sharing information that is useful," another US official said.
The BBC's Haroon Rashid in Islamabad says the Pakistani authorities are eager to dispel suggestions by some US officials that it orchestrated the arrest to derail Afghan government efforts to talk with the Taliban.
That charge has been flatly rejected by Pakistani officials.
"They [the Americans] wish to look for controversies where there is none. It was they who led us to arrest him," one told the BBC.
The official said Pakistan has a clearly defined policy to arrest all militants it can find on its soil: "The operative word here is find," he said.
Former United Nations envoy Kai Eide told the BBC soon after the arrest that it had put an end to UN attempts to talk to the Taliban.
Mullah Omar has not been seen in public for years
There were conflicting reports that before his arrest Mullah Baradar had been talking to Afghan President Hamid Karzai.
Pakistan has denied these claims.
But the commander of US and Nato forces in Afghanistan, Gen Stanley McChrystal, described the arrest as a potential game-changing development after eight years of war.
Mullah Baradar is believed to have been second-in-command to the Taliban's reclusive chief, Mullah Omar. He was said to be the main day-to-day commander in charge of attacks - including suicide bombings - against US and Nato troops in Afghanistan.
Correspondents say that many questions remain about his capture - in particular Pakistan's motivations in carrying it out, the intelligence that led to his whereabouts and what prompted the ISI to act against its long-time Taliban allies.
According to Interpol, Mullah Baradar was born in 1968 and served as deputy minister of defence for the Taliban regime in Afghanistan before it was toppled in 2001.
He has been subject to UN sanctions including a travel ban, an arms embargo and the freezing of assets.
Mullah Baradar was reported to have engaged in an e-mail exchange with Newsweek magazine in July 2009, in which he vowed to "inflict maximum losses" on US forces in Afghanistan.
"In every nook and corner of the country, a spirit for jihad is raging," the magazine quoted him as saying.