U.S. President Barack Obama talked to French President François Hollande late Friday and expressed condolences over loss of lives after several attacks in Paris, including one in a concert hall, the White House said in a statement. According to a report by the Associated Press, about 70 U.S. citizens, known to have been in France, were unaccounted for.

At least six attacks were believed to have rocked the French capital and nearby locations Friday, and over 150 people were reported to have been killed. Of those killed about 80 were believed to have died at the Bataclan Theatre in Paris, where attackers reportedly blew themselves up with suicide belts after spraying cafes outside with machine-gun fire. Four attackers were said to have been killed at the theater while about 200 people were injured, 80 critically, in the attacks.

“President Obama spoke by phone this evening with President Hollande of France to offer the condolences of the American people for the horrific terrorist attacks in Paris earlier this evening,” the White House said, in a statement, adding: “The President reiterated the United States' steadfast, unwavering support for the people of France, our oldest ally and friend, and reaffirmed the offer of any necessary support to the French investigation. The two leaders pledged to work together, and with nations around the world, to defeat the scourge of terrorism.”

According to AP, the intelligence officials were not aware of any threats that were made before the attacks occurred. The report added that about 70 Americans, known to have been in France, are unaccounted for, although no U.S. citizen is believed to have died in the attacks. Although no specific group has claimed responsibility for the attack yet, authorities say that different sets of attackers might have been involved.

Meanwhile, the U.S. Embassy in Paris advised its personnel and citizens in Paris to "shelter in place" after Friday’s attacks.

"The U.S. Embassy in Paris advises official U.S. personnel and U.S. citizens to shelter in place due to the terrorist attacks in the 10th and 11th arrondissements (districts) and the ongoing hostage situation at the Bataclan Theatre located at 50 Voltaire Boulevard in the 11th arrondissement," the U.S. Embassy in Paris said in a statement, adding: "U.S. citizens should heed local authorities and maintain security awareness."

According to CNN, which cited Paris prosecutor spokeswoman Agnès Thibault-Lecuivre, eight attackers were killed, with seven of them dying in the suicide bombings.

In an earlier address Friday, Obama called the attacks an "outrageous attempt to terrorize innocent civilians," and added that "this is an attack not just on Paris, not just on the people on France, but an attack on all humanity and the universal values we share."