One of the Canadian soldiers was injured, according to the Nato-led international force (IASF).
Canada has said it will end its mission in Afghanistan unless Nato sends more troops to the region.
In the attack in Zabul, a roadside bomb exploded close to a vehicle carrying Romanian soldiers on the main road to Kandhar; three of them were wounded.
The IASF also confirmed that two Afghan women and two children were killed after Nato troops came under attack.
The British defence ministry said an investigation had been launched after the incident in Helmand province.
"We deeply regret that this incident happened and do everything we can to mitigate this from happening. This incident is currently under investigation," an official statement said.
Reports in the British media said the incident happened when air strikes were called in by British ground forces who were under fire from Taleban positions.
The issues of civilian deaths in the continuing conflict between Nato forces and the Teleban is a sensitive one for the Afghan government, and President Hamid Karzai has regularly called on troops to take more care.
Meanwhile the UN security council is considering a mandate for a special envoy to Afghanistan who will be asked to improve co-ordination between Nato forces and the Kabul government.
|Canada wants Nato to send more troops|
to Afghanistan [AFP]
A briefing from the under-secretary general for peacekeeping said the world community "sometimes has not been sufficiently united" in it's strategy for Afghanistan.
"The UN bears a share of the responsibility for the insufficient co-ordination for the international community," said Jean-Marie Guehenno. "We are working to correct that."
The draft mandate, expected to be adopted by the council on March 20, will speall out the mandate of the new special envoy, Kai Eide from Norway.
Eide was appointed to the post a week ago after President Karsai rejected the UN secretary-general's first choice, British politician Paddy Ashdown.