The attack on Tuesday-Wednesday came as the Colombian government and the Revolutionary Armed Forces of Colombia (FARC) are in peace talks that have been under way in Havana since November 2012 in a bid to end more than five decades of conflict that have killed over 200,000 people.
Bogota accused the FARC of committing a war crime by ambushing a resting army unit with unconventional weapons in the attack in the western rebel bastion of Cauca.
Marie Harf, acting State Department spokeswoman, said in a statement: "We reaffirm our continuing support to the government of Colombia in its efforts to end the nation's 50 year conflict.
"We condemn the brutal attack in Cauca orchestrated by the Revolutionary Armed Forces of Colombia (FARC).
"The offensive was in direct violation of the unilateral ceasefire they committed to last December."
Colombian President Juan Manuel Santos lifted a suspension of air strikes on the rebels in the wake of the attack but did not call a halt to the peace talks, AFP reported.
"It is our sincere hope that negotiators reach an accord soon to bring peace to all Colombians," Harf added.