CANBERRA - The deaths of two Australian soldiers in a single bomb blast in Afghanistan have reopened debate on the nation's involvement in a war that is expected to become even bloodier this year.

As the bodies of Sappers Jacob Moerland, 21, and Darren Smith, 26, began their journey home yesterday, Prime Minister Kevin Rudd's Labor Government came under fire for refusing to set a deadline for withdrawal.

And among supporters of Australia's commitment to what Canberra regards as the frontline against international terrorism, the Government was attacked for not providing enough troops to complete the job they were sent to do.

The debate goes on against expectations of an increased tempo of Taleban operations - with the possibility of further Australian casualties - and of the still-unresolved question of replacements for the Dutch troops who have been leading the Nato forces in Oruzgan Province, where the Australians are largely based.

The 1500 Diggers in the province include SAS and commandos, and a large mentoring force readying a local Afghan National Army brigade to eventually take over regional security.

Sappers Moerland and Smith were with a patrol from the mentoring task force when they were killed by an improvised explosive device on Monday, bringing to 13 the number of Australians killed in the war. A further 126 have been wounded in action.

The Defence Force chief, Air Chief Marshal Angus Houston, told a Senate committee there was "growing evidence" the Afghan troops were moving towards the goal of independent operations and they had shown "considerable resilience under fire".

Australia is also increasing its civilian aid to the region. But there is no indication yet that the end is in sight.

Houston told the Senate that while allied forces had checked the momentum earlier gained by the Taleban, "there is still a long way to go in meeting the challenge associated with completing the counterinsurgency campaign".

The warm months ahead are also expected to see violence increase as allied forces boost their operations in the Kandahar region against stiff opposition from the Taleban, who will also use their resistance to undermine support for the war and weaken the struggling Administration of President Hamid Karzai.

Defence Minister John Faulkner warned on ABC radio yesterday that the next few months could be particularly violent: "I can only say to you that Australian troops ... are at great risk.

"This is something that I just have to be absolutely frank with you and all Australians about."

And Rudd said Australian troops would stay. "Under no circumstances can Australia afford to allow Afghanistan to become a free, operating, training base for terrorist organisations."

But the latest Lowy Institute for International Affairs poll of Australian attitudes showed that despite their strong support for the US alliance, patience is growing thin with the country's involvement in the war.

It found that 54 per cent wanted Australia to pull out, and said the majority of respondents were no longer confident their country had clear aims in Afghanistan.

With the Dutch beginning their withdrawal in August and the Americans planning to start theirs next year, Greens leader Bob Brown yesterday said it was time Parliament debated the nation's military presence there.

A Sydney Morning Herald commentary said the Government's position seemed to indicate its heart was not really in the fight but was there simply to wave the flag, and urged "an honest appraisal" of the mission.

Supporters of the war said Australia did not have enough troops there to complete the task and called for reinforcements, a prospect Opposition leader Tony Abbott has said he would consider if he wins power this year.

But the Government is worried that any increase in its Afghan deployment could seriously erode its ability to handle emergencies in its immediate neighbourhood.

* A Nato helicopter was shot down by hostile fire in southern Afghanistan late last night (NZT), killing four American troops.

Associated Press reported Taleban insurgents shot off two rockets to down the helicopter in the Sangin district of Helmand province.