TUNIS — The plan, hatched for months in the arid mountains of North Africa, was to attack the American and British Embassies here. It ended in a series of gun battles in January that killed a dozen militants and left two Tunisian security officers dead.

But the most disturbing aspect of the violence in this normally placid, tourist-friendly nation is that it came from across the border in Algeria, where an Islamic terrorist organization has vowed to unite radical Islamic groups across North Africa.

Counterterrorism officials on three continents say the trouble in Tunisia is the latest evidence that a brutal Algerian group with a long history of violence is acting on its promise: to organize extremists across North Africa and join the remnants of Al Qaeda into a new international force for jihad.

[Last week, the group claimed responsibility for seven nearly simultaneous bombings that destroyed police stations in towns east of Algiers, the Algerian capital, killing six people.]