Blasts may be start of campaign: experts

Deadly blasts that rocked two Jakarta hotels on Friday could be the start of a continued bombing campaign in Indonesia, experts say.

Two Australians - Perth businessman Nathan Verity, 38, and senior trade official Craig Senger - are believed to be among those killed in the explosions at the Ritz-Carlton and Marriott hotels.

Prime Minister Kevin Rudd says he has grave concerns for three Australians.

Carl Ungerer, national security project director for the Australian Strategic Policy Institute, said Indonesia's most wanted terrorist, Noordin Mohammed Top, or people acting for him, were likely to be behind the bombings.

"The threat from a continuing campaign of bombing remains high, particularly in Indonesia and as new recruits join the organisation," he told ABC Television.

"As they gravitate towards this very hardline view, this splinter group, then I do think we can expect to continue to see this sort of thing."

International Crisis Group senior adviser Sidney Jones said the bombings were likely to have involved 10 to 15 perpetrators.

"This is the most sophisticated (bombing) that we have seen in a long time, I think, both in terms of coordination and the targeting," she told ABC Television.

Indonesian Police Chief Bambang Hendaro Danuri told reporters in Jakarta the heads of two suicide bombers had been recovered.

He said police were still working to identify the bombers and the victims.