However, no specific links had been uncovered yet between terrorists and the Pakistani government, the unnamed official said, according to the 'New York Times'.
"A former Defense Department official said on Wednesday that American intelligence agencies had determined that former officers from Pakistan's Army and its powerful Inter Services Intelligence agency helped train the Mumbai attackers," the paper said.
It, however, did not identify the official, saying he had spoken on condition of anonymity.
The disclosure came as Secretary of State Condoleezza Rice held meetings with Indian leaders in New Delhi and Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, met their Pakistani counterparts in Islamabad, in a two-pronged effort to pressure the country to cooperate fully in tracking down the perpetrators of the bloody attacks, the paper said.
Pakistan has refuted Indian allegations that militants operating from its soil were responsible for the deadly attacks, with President Asif Ali Zardari terming them as "non-state actors".
US tells Pak to 'investigate aggressively' Mumbai attack links
The United States has asked Pakistan to "investigate aggressively" all possible links between Pakistani groups and the Mumbai attacks and to take more action against militants across the country.
Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, asked Pakistan's top leadership to "investigate aggressively any and all possible ties to groups based in Pakistan", the US embassy said in a statement.
While taking note of the recent success of Pakistani security forces in operations against militants on the Afghan border, Mullen "also encouraged Pakistani leaders to take more, and more concerted, action against militant extremists elsewhere in the country", the statement said.
"All agreed that the tragedy in Mumbai represents a dangerous escalation in the sophistication of extremist attacks and an increased threat to the entire region," the statement said.
Mullen, who is on a two-day visit to Pakistan for talks aimed at defusing the tensions with India after the Mumbai attacks, met with President Asif Ali Zardari, Mahmud Ali Durrani, Advisor to the Prime Minister on National Security, Gen Tariq Majid, chairman of the Joint Chiefs of Staff Committee, army chief Gen Ashfaq Parvez Kayani and ISI chief Lt Gen Ahmed Shuja Pasha.
His visit to Pakistan coincided with a visit to India by US Secretary of State Condoleezza Rice, who is expected to travel to Islamabad on Thursday.