Two days after Islamist militants abducted an undetermined number of hostages -- including Westerners -- at a gas plant in a remote section of Algeria, "ongoing activity at various locations" was continuing, a British official said Friday.

It was not clear whether that activity represented "mopping up and checking" or "something more active" being carried out by Algerian forces against the abductors, the official told CNN.

Algerian forces launched their operation upon noticing the hostages being moved toward "a neighboring country," where kidnappers could use them "as a means of blackmail with criminal intent," Algerian Communications Minister Mohamed Said told state television on Thursday.

The British official said there was a "significant" number of British victims and others were unaccounted for.

British Foreign Secretary William Hague was to release more information Friday from Australia, and a parliamentary statement will be issued, the official said.

The official's account was in line with Prime Minister David Cameron's warning of "bad news ahead" issued late Thursday

Algerian troops fired on at least two SUVs trying to leave the kidnapping site, Algerian radio said, citing local sources. And an Algerian reporter saw clashes near the site, the Algerian Press Service and radio reports said.

"There were a number of dead and injured, we don't have a final figure," Said reported.

Earlier, Algeria's state media reported that all Algerian nationals who had been held hostage were free: some had fled, while others were released. The hostages still detained are foreigners, Algerian Interior Minister Dahou Ould Kablia said.

In addition to the hundreds of freed workers, 30 Algerian workers escaped and were picked up by helicopters, the APS report said.

Nearly 600 workers and four foreign nationals -- two Scots, a Kenyan and a French citizen -- were free by late Thursday after the Algerian military operation, the state-run Algerian Press Service reported.

But some hostages were still presumably being held. "It is a fluid situation, it is ongoing," Cameron told the Reuters news agency. "I think we should be prepared for the possibility of further bad news, very difficult news, in this extremely difficult situation."

The Algerian military operation led to numerous casualties and was over by Thursday evening, the Algerian Press Service said.

But U.S. and British officials predicted it would resume Friday during the day.

"There are still hostages, and there are still terrorists," a senior U.S. official said. "So tomorrow is another day."

Two people -- an Algerian and British national -- died Wednesday when the Islamist militants attacked, the same news agency said.

The Algerians and foreign workers were taken hostage early Wednesday in an assault carried out apparently in response to France's offensive in neighboring Mali. The gas field is 60 kilometers (40 miles) west of the Libyan border and 1,300 kilometers (about 800 miles) from the Algerian capital, Algiers.

The kidnappers were equipped with AK-47 rifles and put explosives-laden vests on some of the hostages, a U.S. State Department official said.

On Wednesday, the attackers had put the number of hostages at "more than 40," including seven Americans, two French, two British and other Europeans. Another Islamist group told the Mauritanian News Agency there were 41 "Westerners."

The APS, though, reported that slightly more than 20 foreign nationals were being held.