A shadowy, Pakistan-based militant faction is on the rise within the Taliban after its leader was appointed deputy and played a key role in unifying the fractured insurgency.

The ascendency of the Haqqani network, a U.S.-designated terrorist organization, could significantly strengthen the Taliban and herald another summer of fierce fighting in Afghanistan. The firepower it brings to the Taliban was shown by a Kabul bombing last month that killed 64 people, the deadliest in the Afghan capital in years, which experts say was too sophisticated for the insurgents to have carried out without the Haqqanis.

The network's role could also further poison already tainted relations between Islamabad and Kabul. Afghanistan is pressing Pakistan to crack down on the Haqqanis, accusing it of tolerating the group, a charge the Pakistanis deny.

An audio recording of a recent meeting of the top Taliban leadership, obtained by The Associated Press, offers a glimpse into the influence the Haqqani network now holds within the movement. Sirajuddin Haqqani, the network's leader and newly elevated deputy head of the Taliban, tells the gathering that they must end differences and focus on fighting. "It is time to work. The mujahedeen (Islamic holy warriors) are happily going to the battlefield," he is heard saying. The voice is recognizable as Haqqani's.

Haqqani's rise to the deputy post is the highest, most direct role that the network is known to have taken in the Taliban leadership. The network pledged allegiance to the Taliban years ago but has traditionally operated independently.

The network was founded by Jalaluddin Haqqani, a one-time ally of the United States who achieved fame fighting the Soviets in Afghanistan in the 1980s and who developed close ties to the slain al-Qaida chief Osama bin Laden. After his death, his son Sirajuddin Haqqani took over.

The elder Haqqani aligned his group with the Taliban after the insurgents were driven from power in the U.S.-led invasion that followed the Sept. 11, 2001 attacks. He was a formidable militant financier, traveling to Saudi Arabia and the United Arab Emirates to raise money. The network maintained close ties to al-Qaida and is believed to have large numbers of Arab and other foreign fighters.

The network is believed to command thousands of fighters on both sides of the Afghan-Pakistani border. Over the years, the Haqqanis emerged as the Taliban's strongest asset because of their battle-hardened fighters and traditional links to Pakistan's security agencies. Both U.S and Afghan intelligence agencies say Pakistan's intelligence network, known as ISI, has allowed the Haqqanis to live freely for decades in Pakistan's tribal regions, a claim Islamabad denies.

"There's no one sole source of the Haqqani network's strength, though three places you can point to are its personnel, its links to Pakistan, and its ties to the Gulf region," said Michael Kugelman, senior associate for South and Southeast Asia at the Woodrow Wilson International Center for Scholars in Washington.

The Taliban leadership meeting, held early last month at an undisclosed location, focused on strategies and battles ahead, the audio recording shows. During the meeting, Sirajuddin Haqqani called on the Taliban to close ranks, reminding them their enemy is the "foreign infidel" and not each other.

"Our objectives should be service to religion and we should end our differences and complaints," he says in Pashtu, the language of Afghan and Pakistani Pashtuns, the ethnic group that makes up the backbone of the Taliban movement.

Since last summer, Sirajuddin Haqqani has been instrumental in reconciling differences among Taliban commanders, who balked at recognizing Mullah Akhtar Mansoor as the supreme Taliban chief following the announcement that the insurgency's reclusive leader, Mullah Mohammad Omar, was dead.

As soon as Mansoor became leader, he announced that he had named Haqqani as his deputy.

Haqqani quickly set about uniting the fractured Taliban, first by bringing Mullah Omar's son, Mullah Yaqoob, and his brother, Mullah Abdul Manan Omari, into the fold, according to a Taliban official who spoke to the AP on condition of anonymity because he's not authorized to talk to the press.

Haqqani then coaxed Zakir Qayyum, a Taliban strongman in the battleground southern Afghan province of Helmand and the former head of the Taliban military committee under Mullah Omar, to swear allegiance to Mansour, healing some of the biggest divisions within the Taliban, the Taliban official said. Fahd Humayun, program and research manager at the Jinnah Institute, a think-tank in the Pakistani capital, who closely follows Taliban developments, also said Haqqani was key to healing the divisions.

The rise of the Haqqanis comes at a critical juncture in relations between Kabul and Islamabad.

In the off-and-on effort for negotiations over the years, Pakistan has hosted past meetings between the Kabul government and the Taliban, whose leaders are widely believed to be based in the Pakistani cities of Quetta and Peshawar, near the border with Afghanistan.

A four-nation group that included Pakistan, Afghanistan, China and the United States launched efforts earlier this year to try to bring Afghanistan's protracted war to a negotiated end but the push fell apart amid recent Taliban battlefield gains.

Afghan President Ashraf Ghani now says he is no longer interested in having Pakistan bring the Taliban to the negotiating table. Instead, he demands Islamabad stop harboring the Taliban and take up the fight against the Haqqanis.

The U.S. State Department has also repeated its demand that Pakistan take action.

"We have consistently expressed our concerns at the highest level of the government of Pakistan about their continued tolerance for Afghan Taliban groups, such as the Haqqani network, operating from Pakistani soil," U.S. State Department spokesman Elizabeth Trudeau said at a press briefing following the Kabul attack.

Islamabad claims it decimated the Haqqani network's infrastructure in a military operation launched two years ago in North Waziristan, where the Haqqanis are headquartered. It points to its own grievances with the Haqqanis, who also maintain links with the Pakistan Taliban, a separate group that has killed hundreds of Pakistani soldiers in recent years.

Islamabad has also aided the United States in its drone strikes against the Taliban and other militants by providing ground intelligence for the Americans, said Humayun of the Jinnah Institute, though officially Pakistan condemns the strikes because of civilian deaths.

While Pakistan pledged to dismantle militant networks on its territory, Pakistan's special adviser on foreign affairs, Sartaj Aziz, earlier this week told reporters in Islamabad that the only solution to the Afghan war was through peace talks.

Should the Haqqanis be pushed out of Pakistan and back into Afghanistan, Ghani's government and Afghan forces would likely be overwhelmed, say analysts.

"I can't imagine the Afghans would be able to take on the Haqqanis themselves," Kugelman said. "I'm not sure they'd be able to take them on even with assistance from foreign combat forces."