Washington has handed over specific demands
Washington has handed over a list of specific requests to Islamabad - an ally of Afghanistan where prime suspect Osama bin Laden is believed to be hiding.
The BBC's Adam Mynott in Islamabad says that there is some suggestion that the US requests might also involve some presence on the ground.
Any action against Afghanistan by US forces would be virtually impossible without such co-operation from Pakistan, correspondents say.
Pakistan's military leaders condemned the attacks in New York and Washington, but did not spell out how far they were ready to go.
Musharraf's dilemma
Earlier this week, Pakistan's president Pervez Musharraf made a general pledge to help, and US Secretary of State Colin Powell said they had a "positive" telephone conversation on Thursday.
Our correspondent says, however, that while many are appalled by the brutal attacks on the US, they may find it impossible to support Pakistani involvement in action against their neighbour.
Hardline Islamists have already warned of a public backlash if any such move was made.
"If the government allows Pakistan to be used for attacks on Afghanistan it would be a great treachery," said Maulana Samiul Haq, the leader of an umbrella group of religious political parties and Islamic militant groups.
And a former intelligence chief, Hamid Gul, said it was not in Pakistan's interests to support the Americans against the Afghans.
Strong ties
At the same time, Pakistan is feeling the weight of economic sanctions imposed by the US over 1998 nuclear tests, which include bans on foreign aid and other financial assistance.
Mr Powell has reportedly hinted that the measures, and the prospect of their abolition, could be used to press Pakistan to meet US demands.
Pakistan is one of the few countries that acknowledge the rule of the strict Islamic Taleban regime in Afghanistan.
Given Pakistan's close ties with the country, the US has long been urging Islamabad to push the Taleban to expel Mr Bin Laden.
Islamabad is also thought to have the best intelligence on Afghanistan - information that would prove crucial in any American action on that country.
On Friday, the chairman of the US Senate Intelligence Committee, Joseph Biden, told CBS news that Pakistan would have to make a commitment soon.
"They've got to choose sides," Mr Biden said.
"There is no question. You're either our friend or you're our enemy here," he said.