By M Ilyas Khan
BBC News, Islamabad
Some strong anti-American sentiment is expressed in Pakistan
US diplomats have complained that Pakistan is delaying the extension of visas of more than 100 US officials.
They say that those affected are engaged in diplomatic, military and aid services and may not be able to return to Pakistan after the Christmas break.
There are also reports of US diplomatic vehicles being repeatedly stopped and searched at Pakistani checkpoints.
But Pakistani foreign office spokesman Abdul Basit told the BBC there was "no question of delaying tactics" on visas.
The claims come at a time when the US administration is calling for international support for its "war on terror".
The latest developments indicate a new low in US-Pakistan relations.
"At least 135 visas are being held up by the Pakistani authorities," the chief spokesman of the US embassy in Pakistan, Richard Snelsire, told the BBC News website.
More and more US officials are applying to go to Pakistan
"Extensions of these visas have not been denied, but they are being delayed," he said.
But Mr Basit said that Pakistan had good relations with the United States, so there was no question of any deliberate initiative to inconvenience the Americans.
"The delays may be due to procedural constraints," he said.
"It happens in other countries as well. Many Pakistanis sometimes get visas very late, or are refused."
The Associated Press news agency quoted an unnamed US diplomat as saying the visa clampdown and the US vehicle searches were a reaction to widespread anti-American sentiment in Pakistan, and were probably temporary.
American officials say that those employees likely to be affected by the visa delays include aid workers and others bringing help to Pakistan.
US President Barack Obama signed into law a $7.5bn (£4.6bn) aid package for Pakistan last October for economic and social programmes.
Under the law, a large number of American workers must be deployed in Pakistan to carry out accounting and aid-monitoring procedures.
American diplomats believe this additional deployment may increase the strength of the US embassy staff from about 500 to nearly 800.
Various quarters in the Pakistani establishment have been critical of this impending increase in US diplomatic activity in the country.
The Pakistani army has also publicly criticised some provisions of the US aid package as "intrusive meddling" in the country's internal affairs.
In recent weeks, the Pakistani media has carried prominent reports of incidents of American officials "resisting" Pakistani law enforcement officials conducting searches of their vehicles at security checkpoints.
The Americans believe the practice is meant to harass US diplomats.