The bombing of the Islamabad Marriott is a warning by al Qaeda and the Taliban that Pakistani and US attempts to eradicate militant hideouts will be met with unprecedented bloodshed, analysts say.
There has been no claim of responsibility for Saturday's attack, which killed at least 60 people and left the luxury hotel in flames, but security officials pointed the finger at the allied Islamist extremist groups.
The militants are under pressure from a Pakistani offensive near the Afghan border and from a rash of US missile strikes, but the Marriott blast will put the fragile Pakistani government's willpower to the test.
"The attack is a message from al Qaeda and Taliban that unless Pakistan and America stop attacking their sanctuaries in the tribal areas, they can hit back in Pakistan if not the United States," analyst Hasan Askari told AFP.
Hundreds of militants have been killed in recent weeks the lawless tribal region of Bajaur and in the scenic Swat valley, until last year a popular tourist destination for its mountains and Buddhist ruins.
The offensives had caused "unnerving damage" to the militants and made them "furious", said Askari, who teaches at Johns Hopkins University in the United States.
Osama Bin Laden's deputy Ayman Al-Zawahiri, who narrowly escaped a US missile strike in Bajaur in 2006, is known to have a network in the region, while bin Laden himself is believed to be somewhere in the border zone.
But analysts said they expected the militants to intensify their efforts to fight back, mainly by exploiting the economic and political problems of the new civilian government-as well as a growing rift with the United States.
The Marriott has long been a symbolic target for attacks by Islamist extremists, drawn by the fact that it is the capital's main meeting place for western diplomats and the Pakistani elite.
A security official said the attack bore the hallmarks of al Qaeda and had similarities to the bombing of the Pakistani federal investigation agency headquarters in the eastern city of Lahore earlier this year.
US-allied President Asif Ali Zardari, who made his maiden address to parliament just hours before the blast, vowed after the attack that the government would fight the "cancer" of terrorism.
But Zardari, whose wife, former premier Benazir Bhutto, was killed in a suicide bombing in December, has failed to set out any detailed plan for tackling the ingrained militant problem along the north-western frontier. His government also faces public anger over breaches of sovereignty by US troops based in Afghanistan, including a commando raid into Pakistan early this month, which have further undermined support for the "war on terror".
"It is a clear warning to the government and the military how things are going to be in future if there is any sustained campaign against militants in the north-western region," Talat Masood, a security analyst and retired army general, told AFP.
He said the country's new civilian leadership, which toppled President Perfuse Musharraf in August, lacked a clear understanding of the situation and was repeating Musharraf's mistakes of relying on military might.
"The government has to come up with a workable strategy combining political, military and development aspects to deal with the crisis. Otherwise the situation will get worse and worse," he said.
Many in Pakistan also pointed accusing fingers at the apparent security lapse which led to the Marriott bombing, after the government said that authorities had had an intelligence tip-off about a possible terror attack. "It is mind-boggling that a truck with up to 600 kilogrammes of explosives roamed around undetected amid a red security alert because of Zardari's address to parliament," a Pakistani security official said.