OTTAWA, July 28 (Reuters) - Canada will investigate why its troops opened fire on a car in southern Afghanistan, killing two small children, Defence Minister Peter MacKay said on Monday.

He noted, however, that earlier probes into similar civilian deaths had always absolved the soldiers involved.

Military officials say a two-year-old boy and his four-year-old sister died when the troops blasted a car approaching their convoy in Kandahar province on Sunday. They said the driver had ignored several warnings to stop.

MacKay said Canadian soldiers were working in "tough, trying and intense circumstances" and often had to deal with attacks from Taliban militants hiding among civilians.

"When these tragic circumstances arise, we always take the important time to investigate what took place. There are rules of engagement followed by Canadian, followed by NATO soldiers," he told reporters in Ottawa.

"In each circumstance where this type of very unfortunate tragedy occurs, Canadians have been found to have followed those rules of engagement," he said.

Canada has 2,500 soldiers based in Kandahar on a mission that is due to end in 2011. So far 88 have died.

Speaking earlier in the day, MacKay said Canada would send an extra 200 troops to Afghanistan to maintain and operate unmanned aerial surveillance vehicles and helicopters. (Reporting by David Ljunggren; Editing by Frank McGurty)

This video is from CBC, broadcast July 29, 2008.

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