The attacks by gunmen who killed at least 129 in bloody rampages across Paris bore the hallmarks of Islamic State, the militant group that controls parts of Syria and Iraq, Deputy National Security Adviser Ben Rhodes said.
"First of all, we're clearly going to work very closely with the French in terms of intelligence sharing, also in terms of their military response inside of Syria," Rhodes said on ABC's "This Week" talk show.
US intelligence is supporting French efforts to "identify anybody connected at all with these attackers" and determine whether any are still on the loose, he added in a separate interview with CNN's "State of the Union."
If France decides to invoke Article V of the NATO treaty, which states that an attack on one member is an attack on all, the United States would "absolutely" support it, Rhodes said.
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"We stand ready to do whatever is necessary to support France in this time of tragedy," he said.
"We're looking how we can take efforts with France in the days ahead to ensure there's justice in this attack and make sure there's not a safe haven.