Australia's Federal Police Commissioner Mick Keelty says a second cell of militants may be active in Indonesia and could be poised for another attack.
Police said this vehicle carried the bomb
The warning comes a day after an attack on the Australian embassy in Jakarta that killed at least nine people.
Mr Keelty said the two cells were believed to have been recruited by militant leaders wanted in connection with the 2002 Bali nightclub bombings.
He was speaking in an Australian Broadcasting Corporation interview.
Mr Keelty, who was accompanying Australia's Foreign Minister Alexander Downer on a trip to Jakarta, made his comments shortly before boarding a plane to return to Australia.
"There is intelligence about a second group," he said. "It's uncorroborated, but you can't discount any intelligence that you get at this point in time."
Earlier, Mr Downer pledged to act against the perpetrators of Thursday's "cruel and callous" bomb attack, in which more than 180 people were injured.
"Our officials will do everything they can to help the Indonesians hunt down the people responsible for this brutality," he said.
Indonesian police say one or more militants may have died in what they believe was a suicide attack using a mini-van.
Police said the attack bore the hallmark of Jemaah Islamiah, the militant Islamist group accused of a series of attacks, including the bombing of two nightclubs on the Indonesian holiday island of Bali in October 2002.
Mr Downer told a news conference that police may have received a text message shortly before the embassy blast. The warning is said to have demanded the release from of Islamic cleric Abu Bakar Ba'asyir who is being held on suspicion of involvement in terrorism.
But Indonesian police denied they had received such a warning.
Ba'asyir, who is currently in police detention, is accused of being JI's spiritual leader.
The embassy attack has prompted fears that militants could be trying to influence the outcome of Australia's general election on 9 October.
Prime Minister John Howard reacted by saying the country would not be "intimidated by acts of terrorism".
Terrorism and the conflict in Iraq are key issues in what is expected to be a tight race between Mr Howard - who is seeking a fourth term - and the Labor Party leader Mark Latham.
If he wins, Mr Latham has pledged to withdraw Australian troops from Iraq.
The blast is also likely to overshadow Indonesia's presidential election on 20 September. Incumbent Megawati Sukarnoputri and challenger Susilo Bambang Yudhoyono have both visited the scene of the attack.