Defence Secretary Geoff Hoon has hit back at critics who say UK troop deaths in Iraq will spark new public anger.
Three Black Watch members were killed in Iraq
Three Black Watch soldiers and a civilian interpreter were killed on Thursday in a suicide attack.
Scottish National Party leader Alex Salmond says the soldiers' bravery contrasted with the "chicanery" of the politicians who had sent them there.
Mr Hoon accused Mr Salmond of sinking to new depths and insisted the Black Watch mission was not impossible.
And paying tribute to the Black Watch, Tony Blair said the regiment was doing a crucial job to prepare for Iraqi elections.
At a news conference with Iraqi interim Prime Minister Iyad Allawi, Mr Blair expressed his deep sympathy and condolences to
the families of the dead soldiers.
He also voiced his "pride and gratitude to the Black Watch for the extraordinary and
heroic job they are doing there, which is of crucial importance to making sure
democratic elections can go ahead in Iraq".
The continued training of Iraqi forces would be a priority, he added.
The deployment of the 850-strong Black Watch force into the US-controlled zone close to Baghdad was criticised by MPs on all sides.
Many were concerned they were moving into a more dangerous area of operations south of Baghdad to free up US forces for an assault on the troubled insurgent-held town of Falluja.
The deaths of Sgt Stuart Gray, Pte Paul Lowe and Pte Scott McArdle came just hours after the Black Watch force extended their area of operations to the east of the River Euphrates, south of the Iraqi capital.
Troop deaths are usually greeted only with condolences by MPs but the SNP have already been outspoken in their criticisms.
Mr Salmond said the troops' move had clearly been politically-motivated and linked to the American presidential elections.
Grief over the deaths would give way to "a wave of anger as Scotland and Black Watch families compare and contrast the bravery of our Scottish soldiers with the duplicity and chicanery of the politicians who sent them into this deployment", he predicted.
Mr Hoon said anger over the deaths was unjustified.
"I'm afraid the leader of the Scottish nationalists' comments demonstrate there are clearly no depths to which he will not sink," he told BBC Radio 4's Today programme.
The defence secretary described the killings as a "terrible incident".
But he stressed the US request had been made by military chiefs and had not been discussed with his American counterpart Donald Rumsfeld.
He said he did not accept that the Black Watch's job was "impossible" and he was enormously proud of the force.
"This job was looked at very carefully by senior military officers," he stressed, saying there had been no political interference in the advice.
Nicholas Soames said other troops would redouble their efforts
Mr Hoon also denied claims there had been "mission creep" when the troops extended patrols to the east of the Euphrates.
The move was needed to halt rocket attacks at Camp Dogwood from across the river, he explained.
Mr Hoon said there would be a "change in tempo" for the Black Watch soldiers if there was an assault on Falluja but they were used to fending off regular attacks from their time in Basra.
Earlier, Conservative shadow Defence Secretary Nicholas Soames said the latest troop deaths marked a "very bad day" and he extended his "deepest condolences".
He said he was sure the battalion members would redouble their efforts in the task ahead.
Liberal Democrat leader Charles Kennedy said it must be a "worrying and alarming time" for soldiers' families.
His party's defence spokesman, Paul Keetch, added: "Whatever our views on the war in Iraq, our armed forces are performing a difficult job under very difficult circumstances; our thoughts are with them."