40 Commando are preparing to go to Afghanistan
The warning came as US bombs reportedly hit three houses on the outskirts of Kabul on Sunday, killing at least 10 civilians.
Foreign Secretary Jack Straw also said the Afghanistan campaign could last "indefinitely", and said the possibility of halting the bombing raids during Ramadan, which starts on 17 November, was being considered, but was by no means certain.
The prime minister's spokesman Alistair Campbell said Tony Blair believed the war on terrorism would take "both time and patience", and it was vital not to lose sight of the horrors of the attacks on 11 September.
Echoing earlier government concern over the coverage of the bombing raids, the spokesman said "while it was easy to sit on the sidelines and judge, the job will be done".
He added: "We do have a clear strategy". He said Mr Blair had always seen the process as a "long haul and a hard grind".
Downing Street said that there was similar criticism in the Kosovo campaign, but pointed out that the Nato alliance held together and achieved its objectives.
Mr Campbell visited Washington last week to "coordinate messages" and Tony Blair will be speaking to President Bush again later on Sunday.
Call to 'hold nerve'
Earlier, foreign secretary Jack Straw said people had to realise that the war would go on longer than a matter of weeks and that Britain was in "for the long haul".
Mr Straw said he understood that when mistakes were made, it "caused people to worry severely" about the action being taken, but that a campaign was essential to prevent a repeat of the attacks on New York.
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 seek to boost public support for the UK's involvement in the campaign.
On Friday US Navy fighters and B-52 bombers mistakenly bombed six warehouses used by the International Committee of the Red Cross (ICRC), destroying food.
And on Thursday the United Nations confirmed that nine people had been killed when a US cluster bomb landed near a village in western Afghanistan.
But on Tuesday Mr Blair will say it is important that Britain "stays the course" in backing the US-led action.
He will also say the military campaign has had significant results in destroying al-Qaeda terror training camps and undermining Taleban military defences.
"Whatever our faults, Britain is a moral nation with a strong sense of right and wrong," Mr Blair will say.
"And that moral fibre will defeat fanaticism, terrorists and their supporters."
Elusive Bin Laden
A total of 200 troops from 40 Commando have been committed by the UK to ground offensives in Afghanistan.
The UK marines on standby will form part of a force of 4,200 personnel supported by a substantial amount of hardware.
Earlier on Saturday, Defence Secretary Geoff Hoon said it could be a week before marines went into action but that they were raring to go.
However he revealed that the coalition forces still had no idea where Bin Laden was.