A U.S. law enforcement official monitoring the situation in London says at least 40 people have been killed in the explosions in London, reports the AP.
The official spoke on condition of anonymity because British officials have yet to make public the death toll, but U.S. officials learned of the number from their British counterparts.
U.S. authorities were monitoring the situation. The official said it was too early to say who was responsible for the blasts, and investigators were trying to determine whether an al-Qaida cell's claim of responsibility was credible.
The official said there has been no credible, specific threat of an attack in the United States, but that analysts were sifting through recent intelligence for any evidence of follow-on attacks in the USA or abroad, the official said.
The U.S. Homeland Security Department asked authorities in major cities Thursday for heightened vigilance of major transportation systems after a series of explosions on London transit systems.
Italy's European commissioner called a series of explosions in London a terrorist strike against Europe, reports the AP.
Prime Minister Tony Blair said Thursday that it was "reasonably clear" that terrorists were behind a series of explosions in London and called the attacks "barbaric."
Russian President Vladimir Putin taking part in G8 summit in Gleneagles condemned acts of terror in London.