Commandos will stay in Oman for more exercises
Brigadier Roger Lane said he was concerned a lack of intelligence information in the region could hamper their mission.
Speaking to the BBC, he stressed his commandos' role against the Taleban and al-Qaeda forces would be "intelligence-led".
"We will be ready when suitable targets have been identified," the head of 3 Commando Brigade said.
"We do not want to be too hasty."
A total of 200 troops from 40 Commando, based in Taunton, Somerset, are on "immediate standby" to join ground offensives in Afghanistan.
B Company will form part of a force of 4,200 personnel supported by a substantial amount of hardware.
But the forces will spend at least two weeks conducting "live fire" exercises in Oman before sailing to join Operation Veritas against Afghanistan.
Clear objectives
Junior defence minister Lewis Moonie said there was a "certain impatience" from some quarters to see British troops in action.
"That is not the way it works," he told BBC Radio 4's Today programme
"We do not intend to put our people's lives at risk unless we absolutely have to and it will be in pursuit of clear objectives."
Dr Moonie said it would not be as long as a couple of months before they were deployed.
He said the aims of the war remained clear - "to bring Osama Bin Laden to justice, to destroy his network of training camps within Afghanistan and prevent him waging terror worldwide".
But he said the coalition forces still did not know where Bin Laden was based at present.
More than 20,000 service men and women - the largest deployment of UK forces in the Gulf region in a decade - have been working alongside Omani armed forces in Exercise Saif Sareea II.
Transferable skills
Brigadier Lane said many of the skills from the desert exercises would be transferable to the hostile environment of Afghanistan. But he wanted to make sure the marines were totally prepared before going into action.
"We will make sure we have thought through all the possible outcomes so that every man feels confident about what he is going to do if things go well or, indeed, not according to plan," he explained.
UK foreign secretary Jack Straw has already indicated that the anti-terror campaign could last "indefinitely".
Prime Minister Tony Blair is to call on the country to "hold its nerve" in the fight against terrorism, in a speech to the Welsh Assembly on Tuesday.
Mr Blair will say it is important that Britain "stays the course" in backing the US-led action.
The prime minister's spokesman defended the bombing in Afghanistan which was continuing to degrade the Taleban, preparing the way for ground operations.
But he added the UK was not going to "rush into deploying those forces because the media wants a new story".
He said Brigadier Lane's comments that it might take two weeks to train troops for their new operation were "unremarkable" as it would clearly take time to reconfigure British forces for the operation.