The United States on Thursday defended Turkey's military responses to a Syrian shelling that left five Turks dead, but called for a de-escalation of the situation.

US Secretary of State Hillary Clinton voiced support for Turkey, a NATO ally, when she called her Turkish counterpart Ahmet Davutoglu on Wednesday night following the artillery shelling of Turkey's Akcakale town in Sanliurfa province bordering Syria, which killed a mother and her four children and injured 13 others.

It was the first time that Turkish civilians had been killed by fire from Syria, where government and opposition forces have been engaged in fierce fighting for months following an outbreak of anti-government protests in March 2011.

Syria acknowledged the deadly shelling as a "tragic accident," but Turkey responded by firing on Syrian targets on Wednesday and early Thursday.

"From our perspective, the response that Turkey made was appropriate," US State Department spokeswoman Victoria Nuland told reporters on Thursday at a regular news briefing.

"It also was designed to strengthen the deterrent effect so that these kinds of things don't happen again, and it was proportional," she added.

As the Turkish parliament on Thursday adopted a motion authorizing the government to send troops to a foreign country if need be, US Defense Department spokesman George Little called for de-escalating the situation.

"We hope that this doesn't escalate into a broader conflict. We hope that the situation de-escalates," he said at a regular press briefing.