At a joint news conference in London, British Defense Minister Philip Hammond called the loss of life appalling and unacceptable.
"It is the terrorists that bear the sole responsibility," he told reporters.
According to Algeria's state news agency, special forces stormed the complex Saturday in a "final assault" to end the four-day-old hostage crisis. The report didn't say whether any hostages or militants remained alive, and it didn't give the nationalities of the dead.
Hammond didn't criticize Algeria's handling of the attack directly. But he appeared to reference the increased concern from world leaders about the lack of transparency in Algeria's anti-terror operation.
"We're pressing the Algerians for details on the exact situation and the numbers that have been killed and, if any, the numbers rescued," Hammond said.
"Different countries have different approaches to dealing with these things," he said. "But the nature of collaboration in confronting a global threat is that we work with people sometimes who do things somewhat different, slightly differently from the way we do them ourselves."
U.S. Defense Secretary Leon Panetta said much remains "sketchy" about what happened at the remote Ain Amenas gas field.
"We know that lives have been lost," Panetta said.
"Just as we cannot accept terrorism attacks against our cities, we cannot accept attacks against our citizens and our interests abroad," he added. Panetta said that "those who would wantonly attack our country and our people will have no place to hide."
Panetta was scheduled to return to Washington later Saturday.