U.S. officials say President Barack Obama will press Pakistani President Asif Ali Zardari this week to continue a military drive against Taliban insurgents to prevent a situation in which extremists might threaten that country's stability and nuclear arsenal.  President Obama is scheduled to meet the Pakistani leader and Afghan President Hamed Karzai separately at the White House on Wednesday and then hold a three-way summit on regional security.  

Officials here credit the Pakistani government with taking the offensive against the Taliban in recent days.  But they say the Obama administration will continue to press Mr. Zardari for action to prevent what one official termed a "nightmare scenario" of militants possibly seizing nuclear weapons or facilities.

The comments came on the eve of President Zardari's arrival in Washington for what could be pivotal White House meetings also involving Afghan President Karzai.

U.S. alarm about the situation in Pakistan rose in February when a peace deal between Islamabad and Islamic militants in the Swat Valley north of the capital was followed by an insurgent effort to gain control of neighboring districts.

The Pakistani military launched an offensive to roll back extremist gains after U.S. Secretary of State Hillary Clinton accused Islamabad last month of abdicating to the Taliban.

Briefing reporters on Monday, State Department Acting Spokesman Robert Wood said he believed Pakistani authorities "have gotten the message" about the severity of the threat, but that Mr. Zardari will hear that message repeated in Washington.

"What's important here is that these violent extremists be confronted," said Robert Wood. "The Pakistani government and military realize the threat, and are taking steps to try to address that threat.  And we have to continue, as others have been doing as well, to encourage them to continue to step up that fight.  Because this is an existential threat that the government of Pakistan faces."

The New York Times newspaper reported on Monday that U.S. officials are increasingly concerned about the safety of Pakistan's nuclear arsenal because of the spreading insurgency, including the possibility that militants might sieze a weapon in transit or infiltrate nuclear facilities.

But on that issue, spokesman Robert Wood echoed comments by Admiral Michael Mullen, Chairman of the Joint Chiefs of Staff, that he is "comfortable" that nuclear weapons in Pakistan are secure.

"We're confident that Pakistan's nuclear arsenal will remain out of militant hands," he said. "Command and control of that nuclear arsenal is a high priority for the government of Pakistan.  And Pakistan has been engaged with the IAEA in a very intense discussion.  It's got export control legislation that it has put forward.  So we do not have that concern."

Wood said the physical security around Pakistani nuclear weapons is very solid.  He said he was unaware of any discussion within the U.S. government about possible American intervention in Pakistan in case insurgents threatened nuclear sites.