There is speculation that the 150 SAS soldiers offered by Australia may already have been deployed to the Gulf, and Royal Australian Air Force tankers are on standby.
Sources said yesterday that the further offer of an amphibious command ship and reconnaissance aircraft may include two secret EP-3C Orions, similar to the US spy plane forced down in China in April and used covertly over Indonesia.
Howard was jeered by a small group of anti-war protesters as he arrived for an election campaign meeting in Melbourne, and others demonstrated yesterday afternoon outside the US consulate in Sydney's Martin Pl.
Greens Senator Bob Brown also attacked Australian involvement, and the Australian Federation of Islamic Councils warned that the strikes would heighten both the suffering of innocent Afghanis and the danger of retaliation in Australia.
Howard accepted that the nation was at greater risk. "I don't want to dramatise, but equally I don't want to underestimate or understate the obvious - and that is that all of those who stand with our American friends are potential targets."
Howard warned Australia to prepare for long war. "We hope and pray it will not be long but the reality is that it could be protracted. There will inevitably be the risk of casualties."
Opposition Leader Kim Beazley set aside the rivalry of the election campaign to stand by Howard: "I am at one with my political opponents on this issue of international security."
Canberra last week announced the doubling of its counter-terrorism capacity and the re-establishment of a special response unit, but yesterday introduced further measures. Potential Australian targets include US, British and European companies, affiliates and subsidiaries, airliners and ships, and defence bases and installations such as the US-Australia spy base at Pine Gap, which plays a major role in US operations in the Middle East.
Analysts say that terrorists, possibly including long-term sleepers, could be hiding among Australia's large migrant communities, already the focus of counter-terrorism teams from the Australian Federal Police and the Australian Security Intelligence Organisation.
Yesterday the US Embassy in Canberra was cordoned off, security was increased at other US, British, European and Pakistani diplomatic and commercial buildings, and Transport Minister John Anderson said airport security had been raised to even higher levels. Defence Minister Peter Reith said other, secret, measures had also been taken.