"Those who fell on Wednesday were far from home and close to the enemy, doing the hard work that must be done to protect our country from terrorism," Panetta said in a memo to his employees.
"We owe them our deepest gratitude, and we pledge to them and their families that we will never cease fighting for the cause to which they dedicated their lives -- a safer America," Panetta said.
The Taliban has claimed responsibility of the attack in which one of its suicide bombers in the guise of an Afghan soldier entered the heavily guarded outpost in the eastern Khost Province of Afghanistan.
The suicide attacker detonated himself on Wednesday at a forward American operating Base Chapman in Khost Province near the Afghan border with Pakistan, an area believed to be controlled by Taliban warlord Jalaludin Haqqani.
Eight Americans were killed in the attack on Wednesday while six others were wounded. Flags at the CIA headquarters in McClean Virginia would be flown at half-staff in honour of those dead.
"Yesterday's tragedy reminds us that the men and women of the CIA put their lives at risk every day to protect this nation," Panetta said.
"Throughout our history, the reality is that those who make a real difference often face real danger," the CIA director said in his memo.
The US had recently set up the CIA base in the Khost Province to launch Predator drone attacks targeting the Haqqani network, whose Taliban fighters operate on both sides of Afghan-Pakistan border.
