"It is too early for us to come to judgments about the precise details or origin of the attack but it is a terrible reminder of the intentions of those who wish us harm. As you would expect, we are offering our French colleagues our full support as they respond," MI5 chief Andrew Parker told reporters.
A representative from the prime minister's office claimed earlier that the United Kingdom would tighten security measures and border checks and had already sent police officers from anti-terrorist units to France.
A terror attack on the office of satirical magazine Charlie Hebdo by alleged Islamic extremists claimed the lives of 12 people and injured 11 others. A nationwide manhunt has been launched for the gunmen. Brothers Said and Cherif Kouachi, French nationals of Algerian origin, are the main suspects.