Secretary of State Hillary Rodham Clinton stepped up the rhetorical pressure on Pakistan Thursday, warning Pakistani officials that there would be a “very big price” if they do not take action against militant groups staging attacks in Afghanistan.

Clinton’s tougher tone was reinforced by a highly unusual delegation of five top-ranking U.S. officials who traveled to Islama­bad to demand aggressive action against the Haqqani network, a Pakistan-based Afghan militant group blamed for the assassinations of Afghan leaders and a high-profile attack last month on the U.S. Embassy in Kabul.

“We will be delivering a very clear message to the government of Pakistan and to the people of Pakistan,” Clinton told reporters during a stop in Afghanistan on her way to the Pakistani capital. “There should be no support, and no safe havens anywhere, for terrorists who kill innocent men, women and children.”

The talks with Pakistani officials lasted four hours Thursday before they adjourned for the night. A senior State Department official described the talks as “extremely frank” and “very detailed.”

On Friday, Clinton met with Pakistani President Asif Ali Zardari and Foreign Minister Hina Rabbani Khar.

“Pakistan has a critical role to play in supporting Afghan reconciliation and ending the conflict,” Clinton said at a joint press conference with Khar. “We look to Pakistan to take strong steps to deny Afghan insurgents safe havens and to encourage the Taliban to enter negotiations in good faith.”

U.S. officials have accused Pakistan of tolerating — and, in some cases, supporting — Haqqani clan members in a string of attacks against U.S. and Afghan forces in Afghanistan, a charge Pakistan denies.

“We should be able to agree that for too long extremists have been able to operate here in Pakistan and from Pakistani soil,” Clinton said Friday.

While insisting that there is a shared responsibility for fighting terrorism, Clinton hinted of consequences for Pakistan if the government does not do more to stop attacks emanating from the Pakistani side of the border.

“No one should be in any way mistaken about allowing this to continue without paying a very big price,” Clinton said. She said Pakistan’s leaders “must be part of the solution, which means ridding their own country of terrorists who kill their own people and cross the border to kill people in Afghanistan.”

Clinton spoke of a growing international effort to squeeze the Haqqani network on both sides of the border, adding that the effort “will be more apparent in the days ahead.”

“It is a fact that they are operating out of safe havens in Pakistan,” Clinton said. “It took a while before we could turn to those safe havens. Now it is a question of how much more cooperation Pakistan can provide in going after those safe havens.”

Relations between Afghan and Pakistani officials have been badly strained after a series of high-profile attacks and assassinations, including the killing on Sept. 20 of Burhanuddin Rabbani, a former Afghan president and the point man for reconciliation talks between the Afghan government and the radical Islamist Taliban movement. After Rabbani’s death, Afghan President Hamid Karzai said peace efforts were useless unless Pakistan was heavily involved.

Although U.S. officials have held talks with representatives of both the Haqqani network and the umbrella Taliban organization headed by Mohammad Omar, the administration has refused to publicly acknowledge the meetings and insisted that such contacts should be “Afghan-led.” On Thursday, Clinton appeared not only to acknowledge them but also to invite the insurgents for more conversation.

“This has now reached the point, in our opinion, where it’s appropriate to begin talking,” she said in an interview with ABC News. “Part of this is to keep pushing as hard as we can on the peace and reconciliation track to see what comes up, to see whether there is a willingness on the part of any of the leadership of these groups to have a serious discussion.”

Clinton arrived in Islamabad late Thursday to head a U.S. delegation that included newly appointed CIA Director David H. Petraeus and Gen. Martin Dempsey, the new chairman of the Joint Chiefs of Staff, as well as Lt. Gen. Douglas E. Lute, a top White House adviser on the war in Afghanistan, and Marc Grossman, the State Department’s special representative for Afghanistan and Pakistan. The visit, like the stopover in Kabul, was kept under wraps because of security concerns.

The size of the U.S. delegation was intended as a signal of unity following the administration’s conflicting messages in recent weeks. After the previous Joint Chiefs chairman, Adm. Mike Mullen, called the Haqqani network a “veritable arm” of Pakistan’s intelligence service, other members of the administration sought to play down his remarks. “That’s not language I would use,” White House press secretary Jay Carney told reporters in response to Mullen’s comments.

The schedule for Clinton’s 24-hour visit was packed with meetings with Pakistani Prime Minister Yousuf Raza Gilani; the army chief, Gen. Ashfaq Kayani; and the leaders of Pakistan’s intelligence service, foreign service and Finance Ministry. State Department officials said the unusual size of the U.S. delegation was intended both to convey the urgency of the situation in Afghanistan and the continued importance of Pakistan as a critical partner in the war against extremist groups.

Both the Obama White House and the George W. Bush administration have repeatedly called on Pakistan to battle various militant factions based in the remote and lawless tribal regions along the Afghan border. In response, Pakistan has launched a series of military campaigns, including one in 2009 that targeted the Pakistani Taliban.

A harder challenge, however, has been the Haqqani network, a militant organization that has traditionally been aligned with elements of Pakistan’s military intelligence apparatus. Some Pakistani officials have sought to retain close ties to the Haqqanis, arguing that the alliance would give Islamabad increased leverage over Afghanistan after NATO troops have left the country.

The Obama administration has dropped its long-standing demand that Pakistan begin an all-out military offensive in the North Waziristan tribal area. Now it is insisting that the Pakistanis target the Haqqani leadership, work with the United States to do it or get out of the way. Last week, the White House authorized CIA drone strikes in the North Waziristan town of Miran Shah, killing what it said was the network’s third-ranking leader.

Despite the sharper tone toward Pakistan, Clinton said both countries share an interest in resolving the conflict, and she called for the adoption of a common strategy she dubbed “fight, talk, build.”

She said Afghanistan and Pakistan must attempt to do all three things at once: They must pursue an aggressive military campaign against the insurgency on both sides of the border, while simultaneously attempting to negotiate with cooperative elements of the Taliban and strengthening the region’s economic foundations.

As suspicion over Pakistan’s links to Islamist militants has grown in Washington, U.S. officials have repeatedly warned the Pakistanis, and members of Congress have threatened aid cuts. But it is unclear what other leverage the United States has to force action from Pakistan, whose cooperation — and highways leading from the Arabian Sea to landlocked Afghanistan — are crucial to the U.S.-led war effort.

The Pakistani military long argued that it was too overstretched to take on the Haqqani network in North Waziristan, and even some U.S. defense officials said the Pakistani army would probably be defeated by what is widely viewed as a fierce militia. But more recently, Pakistan has defiantly insisted that the Haqqani leadership has rebased itself in eastern Afghanistan and that the problem now belongs to NATO forces.

Staff writer Karen DeYoung in Washington contributed to this report.