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John Stillwell, Pool, Associated Press
German Chancellor Angela Merkel, right, speaks during a joint news conference with British Prime Minister David Cameron following their talks at 10 Downing Street, London, Wednesday, Jan. 7, 2015. Cameron and Merkel expressed their condolences and support to French President Francois Hollande after the deadly gun attack on satirical newspaper Charlie Hebdo.

LONDON — The leaders of Britain and Germany jointly called French President Francois Hollande Wednesday to offer their intelligence agencies' support after the deadly gun attack on satirical newspaper Charlie Hebdo.

Prime Minister David Cameron and Chancellor Angela Merkel met in London for talks overshadowed by what Cameron called "an appalling terrorist outrage."

He said he and Merkel were briefed on the attack by Britain's MI5 and MI6 intelligence services and offered Hollande "our full support and any assistance our intelligence agencies can give to the French at this vital time."

Merkel said speaking to the French leader "was a very moving moment."

"We assured him that we will do everything we can in order to help him," she said.

"In this very desperate hour, we stand by the French people."

Three masked gunmen shouting "Allahu akbar!" stormed the newspaper's Paris offices Wednesday, killing 12 people, including its editor. The publication has long mocked religion and has run cartoons of the Prophet Muhammad.

Cameron said the attack was "different from some of the plots we've seen here in the U.K. from so-called home-grown terrorists."

He said Britain's emergency committee, COBRA, would meet Thursday to discuss the shootings. Britain's terrorism level stands at the second-highest rung of "severe," meaning an attack is highly likely.

At a meeting in Downing Street, Cameron and Merkel also discussed the economy, Ukraine and the European Union, an issue that threatens to dominate British politics before an election in May.

With anti-EU sentiment rising, Cameron has promised a membership referendum if he is re-elected. He wants Britain to stay in the EU, but needs reforms to sell the idea to his Conservative party, and to voters — and needs Merkel's support to achieve those reforms.

Merkel said Britain's fate would be for its voters to decide, but "I like having the U.K. in a strong and successful European Union."