Pakistan has confirmed that US military personnel have arrived in the country, but denied yesterday it would be a base for any US-led strikes on neighbouring Afghanistan.
While it declined to give details of the deployment, a government spokesman said no offensive was being launched against Afghanistan from Pakistan.
Confirmation of the arrival of US military came after the most intense night of bombing on Afghanistan since raids began on Sunday night.
Pakistan's military is on stand-by all over the country today as the most violent anti-American protests to date are expected. News of the presence of US forces on Pakistani soil is expected to fuel the rallies planned in all major cities and towns.
Pro-Taliban Islamic groups last night called for a big turnout.
While the government was refusing to say where forces were based, Pakistani media reports have said US marines, aircraft, helicopters and other military personnel had arrived at Jacobabad in southern Sindh province.
They had also arrived at the remote coastal town of Pasni, on the Arabian Sea in Baluchistan province.
Local sources said security around the remote Pasni airport had been increased and commercial flights to the town halted.
Dr Abdul Ghani Ansari, general secretary of the radical Jamiat-e-Ulema Islam group in Jacobabad, claimed the first US aircraft arrived on Tuesday at the city's Shahbaz airbase, used by both military and commercial flights.
Meanwhile it was reported that more than 140 people have been killed as a result of US raids on Afghanistan.
The Afghan Islamic Press said yesterday villages in the eastern city of Jalalabad had been flattened in Wednesday night's attack and 50 bodies had already been pulled from the rubble.
"So far more than 50 bodies have been recovered and the fear is that the number of martyrs will be more than 100," it quoted a Taliban spokesman in the area as saying.
Afghanistan's ruling Taliban furiously denounced the latest US-led air strikes on Thursday, saying bombs had killed 15 people in a mosque and up to 100 others when planes hit a village with bombs.
At the now-daily Taliban news conference in Islamabad, the Taliban ambassador to Pakistan, Mr Mullah Abdul Salam Zaeef, said the real war had yet to begin.
"When Americans enter Afghanistan, there will start the real war," he said of the US strikes.
"America is thirsty for more bloodshed in Afghanistan. The number of casualties is increasing with the passage of time. This is a gift of America to the innocent people of Afghanistan."
The eastern city of Jalalabad was among the worst hit, with 15 people killed when a mosque was hit in the Surkhrod suburb, he said.
Jalalabad has long been surrounded by guerrilla training camps for the al Qaeda group of Saudi-born militant bin Laden.
The most deadly attack could turn out to have been that on the outskirts of Jalalabad, in the Torghar hills.
"Fresh reports have arrived that Americans attacked the Torghar area near Jalalabad," Mr Zaeef said.
"They hit a village of people, 100 people are reported to have been martyred in this act," he said, adding that the dead were civilians.
The casualties were also being counted in Kandahar, power base of Taliban leader Mr Mullah Mohammad Omar, although the diplomat said his spiritual leader himself was said to be safe.
Meanwhile Pakistan said yesterday it might lift its ban on the entry of Afghan refugees, but it warned that it would deport those already inside the country if they joined in violent demonstrations.
The US war on Afghanistan's ruling Taliban movement is presenting Pakistan with the dual problem of a potential surge in Afghans trying to leave their country, and protests on Pakistani streets against the US attacks.
A Foreign Ministry spokesman said Pakistan had sealed its border to new Afghan refugees because it already had a number estimated at two million to three million and wanted displaced Afghans to receive aid at camps inside Afghanistan.