Cameron updated members of Parliament on last week's raid on the remote eastern Algerian gas complex. British energy company BP is among the companies involved with the facility.
Cameron confirmed Monday that three British nationals were among those killed in the attack. Another three are believed to be dead. Nevertheless, the prime minister said, the British government was assured by its Algerian counterparts that "the terrorist incident was over."
Al-Qaida militants were said to have stormed the complex following a French decision to send troops to neighboring Mali via Algerian airspace.
"This attack underlines the threat that terrorist groups pose to the countries and peoples of that region," Cameron said. "More than ever this evolving threat demands an international response."
British Foreign Secretary William Hague said there were concerns that the facility wasn't secured because of a lingering threat from bombs possibly planted at the site.
Algeria was criticized for taking a unilateral decision to respond with force to the hostage crisis last week. Hague, however, said it was "too early to pass judgment" on the response, which was said to have cost the lives of some of the hostages.
Given the remote location of the facility, Cameron said there was uncertainty surrounding the specific details of last week's raid. He added, however, that promoting democracy in the region would go a long way to addressing the regional terrorist threat.
"We must pursue it with an iron resolve," he said.