The prime minister has sent his sympathies to the families of three Black Watch members killed in an Iraq attack, his spokesman has said.
Three Black Watch members were killed in Iraq
Shadow Defence Secretary Nicholas Soames said it was a "very bad day" and also extended "deepest condolences".
Liberal Democrat leader Charles Kennedy said it must be a "worrying and alarming time" for soldiers' families.
The Scottish National Party's Angus Robertson said the deaths would profoundly affect Scotland.
He added that the thoughts of everybody go to the family and friends of those in the regiment which traditionally draws its members from Scotland.
Tony Blair's spokesman said: "The prime minister's thoughts are with the Black Watch and the families of the Black Watch."
The deployment of the 850-strong 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.
They were moved there to free up US forces for an assault on the troubled insurgent town of Falluja.
The deaths 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.
Nicholas Soames paid tribute to the work of the Black Watch
Responding to a statement on the deaths by junior armed forces minister Adam Ingram, Mr Soames told MPs the whole country would want to wish the Black Watch well as they continued to carry out their duties.
In an interview with the BBC later he said he was sure the battalion members would redouble their efforts in the task ahead.
He said it was very unlikely that the battalion would be pulled out of the zone now that they had been moved there.
However, Mr Soames said he was sure that all the "old embers" of the debate would be raked over.
Lib Dem defence spokesman Paul Keetch described the news as "tragic" and sent his condolences to the families of those killed and injured.
"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 tonight."
Roseanna Cunningham, SNP Scottish Parliament member for the home of the Black Watch, Perth, said the news was "utterly horrifying and sadly exactly what we expected and feared would happen."
"I would simply reiterate that it's doubly disgraceful that when Black Watch soldiers are dying in Iraq the prime minister wants to disband the regiment."
But Labour MP and former member of the Black Watch Eric Joyce said he still believed the deployment was right.
"It's at times like these you just have to accept that there's a higher risk," the Falkirk West MP told BBC News 24.
The soldiers were involved in an "absolutely vital" democratisation of Iraq and should be given "unconditional support" at this difficult time, he added.