Instability in Pakistan always has the Pentagon worried about two things above all - the country's nuclear arsenal and the fight against al Qaeda and the Taliban in areas bordering Afghanistan. But US officials are playing down the impact the resignation of President Pervez Musharraf, long seen as a key-US ally, will have on either issue.
As Musharraf's position has weakened in recent months, US officials have strengthened relations with both Pakistan's military leadership, in particular Army Chief of Staff General Ashfaq Kayani, and the government elected earlier this year.
"We enjoy a good ... defence relationship," said Pentagon spokesman Bryan Whitman. "We work very closely with the military, particularly along the border region, and we would hope and expect that would continue." The US military did not have any immediate concerns about the safety of Pakistan's nuclear weapons as a result of the resignation, a military official said.
The official did not elaborate but analysts said Pakistan's nuclear arsenal was under the firm control of the military and that would not change with Musharraf's departure. The impact on efforts to defeat militants in tribal areas in eastern Pakistan is harder to assess.
US officials say those areas are vital to US national security as al Qaeda leaders have regrouped there and may be plotting attacks on the United States while the region also provides a safe haven for insurgents operating in Afghanistan. Al Qaeda leader Osama bin Laden is believed by some US officials to be hiding in that part of Pakistan.
NO LONGER INDISPENSABLE: Pakistan has received more than 5 billion dollar in US funding for counter-terrorism operations. Last November, Deputy Secretary of State John Negroponte told Congress that Musharraf was an "indispensable" ally in the war on terrorism declared by President George W Bush after the September 11 attacks on the United States in 2001.
But Musharraf had appeared increasingly isolated politically at home in recent months. Washington also has been disappointed by Pakistani efforts to tackle the tribal areas since a civilian government came to power after elections in February. The government, largely composed of enemies of Musharraf, negotiated pacts with tribal leaders under which the military would stay out of their areas and the tribes pledged to take on militants.
US military commanders in Afghanistan said those pacts did little to quell militancy and led to more fighters crossing the border to carry out attacks. Some analysts say the resolution of the power struggle between the government and Musharraf may let Pakistan's leadership devote more attention to fighting militancy.
"I'm actually hopeful," said Karl Inderfurth, a professor at George Washington University and a former assistant secretary of state for South Asian affairs. "Resolving the 'whither Musharraf' question may well allow the government in Pakistan to now focus on issues of even greater urgency and immediacy, like what is happening in the tribal areas."
US Representative Duncan Hunter of California said the US military should be prepared to use technology to step up monitoring of the Afghan border and interdict militant fighters as Washington evaluates how much co-operation it gets from Pakistan's new government.
"The new Pakistani government may or may not prove to be as reliable of an ally in the global war on terror as President Musharraf has been. Only time will tell," said Hunter, the top Republican on the US House of Representatives committee on armed services.