At President George W. Bush's direction, Rice is cutting short a European trip to visit India later this week. Attacks spanning three days killed more than 170 people in the Indian commercial capital Mumbai, including six Americans.
Indian leaders pointed fingers at "elements in Pakistan" although it is not yet clear where the well-planned operation originated.
"We share the grief and the anger of the Indian people but of course Americans were also killed in this attack and they were killed deliberately because they were Americans," Rice said during a press conference aboard her plane en route to London. "That makes this of special interest and concern to the United States."
Attackers chose sites representing the city's wealth and tourism, and reportedly sought out Westerners as victims. Rice will see Indian leaders in New Delhi. She does not plan to go to Mumbai.
A previously unknown Muslim group called Deccan Mujahideen - a name suggesting origins inside India - has claimed responsibility for the attacks. But a top Indian police officer said Sunday he believed the attackers were from Lashkar-e-Taiba, long seen as a creation of the Pakistani intelligence service to help fight India in the disputed Kashmir region. The group was banned in Pakistan in 2002 under pressure from the U.S., a year after Washington and Britain listed it a terrorist organization.
Indian leaders have blamed unspecified "elements in Pakistan" for the 60-hour siege during which suspected Muslim militants hit 10 sites across Mumbai, but have not said whether they believe the terrorists had the backing of any state agencies. Pakistan denied it was involved and demanded evidence.
India has repeatedly accused Pakistan of complicity in terrorist attacks on its soil, many of which it traces to militant groups fighting Indian rule in Kashmir. The U.S. has tried to persuade Pakistan to shift its security focus from India, with which it has fought three wars, to Islamic militants along the Afghan border.
The Mumbai assaults raised fears among U.S officials of renewed violence between India and Pakistan. Both nations possess nuclear arms.
Rice said Pakistan's U.S-backed civilian president, Asif Ali Zardari, has pledged to improve relations.
Zardari replaced President Pervez Musharraf earlier this year and has established polite but distanced relations with Washington. Musharraf was a military man and a Bush administration ally against terrorism, but that closeness cost him support at home.
"It's a difficult task for this new Pakistani government," Rice said, referring to the way Islamabad will respond to the attacks next door. "They know this is a time to step up to the task."
On Monday, Zardari said the terrorists had no links to any government and pledged to work for good relations between the two neighbors.
Before she left Washington, Rice spoke Sunday with President-elect Barack Obama. It was their third conversation about India in as many days.
Obama was expected to announce his pick to replace Rice as the top U.S diplomat later Monday. Democratic officials said Sunday that Sen. Hillary Rodham Clinton would be named.
Rice is to go to India on Wednesday.
The White House announced Rice's trip Sunday hours after Bush assured India's leader the U.S. government will put its full weight behind the investigation.