Singh did not say what foreign link might have been involved in the bombings that killed some 60 people.
Earlier today, the Indian leader talked on the telephone with Pakistan's President Pervez Musharraf and said he reminded Musharraf of Pakistan's promise to fight terrorism.
Singh made the comments when Musharraf called him to offer his condolences.
Speaking to journalists in Islamabad, Musharraf condemned the bombings as "a dastardly terrorist attack," and said his country will fully cooperate in any investigation.
Meanwhile, Indian police guarded New Delhi's exit points and prowled airports, rail and intercity bus stations for suspects in what a police spokesman said was one of the biggest-ever manhunts in the capital. A number of people have been taken in for questioning.
A little known Kashmiri militant group has claimed responsibility for the blasts. However, security experts say the Pakistan-based Lashkar-e-Toiba was the prime suspect.