The benefits of sending an extra 3,300 UK troops to Afghanistan will "far outweigh the dangers", Defence Secretary John Reid has said.
Mr Reid said the troops would help reconstruct Afghanistan
Mr Reid was speaking after a senior soldier claimed the move would provide "many more targets for insurgents".
Lt Col Henry Worsley told the Sunday Telegraph it would be like "stirring up a hornets' nest".
But Mr Reid told Sky News it was less dangerous than allowing the country to slip back into the hands of terrorists.
He said British troops were going to help with civilian aid and economic development, and to help Afghan President Hamid Karzai to reconstruct the country.
The majority of the troops will be sent to the south, to the volatile Helmand province.
They will form part of a new, 9,000-strong Nato multi-national brigade, the International Security Assistance Force (Isaf), replacing an existing and smaller US force.
Mr Reid acknowledged that the south would be more dangerous for British troops there than it had been in the north and west where they had been deployed previously.
The mission was getting "riskier" with a "greater propensity" for terrorists to attack as troops moved south and they were forced to retreat into their "last bastions", he added.
He said fewer than 10 British troops had been lost in Afghanistan since 2001 and each of them was "bitterly regretted".
But he added that thousands of people had lost their lives because of terrorist attacks prepared in and launched from the country.
According to Chris Willach, who advises non-governmental organisations on security in Afghanistan, the troops may face threats ranging from small arms fire to suicide bombers and targeted assassinations.
In Helmand, which he calls a "high risk area", there are between two and 10 such incidents per week, he told BBC News.
International Development Secretary Hilary Benn said the operation would be difficult but the troops would be equal to the task.
He told BBC One's The Politics Show: "If people try to take on those forces then they will respond. But they are not going there, as John [Reid] said, to wage war.
"They are there to help support a process of reconstruction, to ensure there is security."
Roads and power
The US ambassador in Kabul, Richard Neumann, says there is still much reconstruction to be done, particularly road-building and improvements to power supply.
He told BBC News the insurgency still continuing in the south was probably not strong enough to stop reconstruction but would continue for some time.
The extra 3,300 UK troops will join the 1,100 already in the country and 1,950 announced earlier, but the total at any time is not expected to exceed 5,700.
Helmand is said to be one of Afghanistan's toughest areas
Mr Reid said it was hoped other countries including Australia, New Zealand and the Netherlands would also send troops to Nato's Isaf mission.
But the Netherlands has been wavering about its commitment, with one of the parties within its governing coalition forcing a parliamentary vote on the matter.
Luce Wies Van der Laan, deputy head of the Democracy 66 Party, told BBC News the mission was "flawed in its conception" because it was "trying to do reconstruction in what is essentially a combat zone".
She said the situation in the south was not stable enough for reconstruction work to be carried out and what was needed was a "serious combat mission".
On Saturday Mr Karzai insisted his government was in charge of the whole of the country.
He said the UK deployment was not an expansion of troops, but would maintain current levels as US troops withdrew.