(CNN) -- The United States launched a series of airstrikes Wednesday against ISIS forces in Iraq, the same day President Barack Obama vowed to act against the militant group following its beheading of an American journalist and its threat to kill another. "The United States of America will continue to do what we must do to protect our people. We will be vigilant and we will be relentless," the President said in televised remarks. Obama vowed justice for James Foley, calling his killing by ISIS, which refers to itself as the Islamic State, an act of violence that "shocked the conscience of the entire world."
@highlight
NEW: United States attempted a rescue in Syria, Pentagon spokesman says
@highlight
More Americans, including Steven Sotloff, held by ISIS, official says
@highlight
"The United States will do what we must," President Obama says
@highlight
Airstrikes resume near Mosul Dam, a day after video of U.S. journalist
The President's statement followed Tuesday's release by ISIS of a video that showed Foley's killing and carried a stark warning that a second American, believed to be journalist @placeholder, would be killed, if the United States did not end its military operations in Iraq.