Blood was splattered across the mosque's main hall and walls, while the building's doors and windows were destroyed and its ceiling fans mangled by the blast, according to an AFP reporter who was on site at the country's deadliest attack in three months.
"Our thoughts and prayers are with the families and loved ones affected by this deplorable violence," Clinton added.
Ball bearings used in the suicide vest were also scattered across the mosque in Jamrud town, 25 kilometers (16 miles) from Peshawar, the main city in the Khyber tribal district where much of the violence in Pakistan is concentrated.
The attack came as a US drone strike killed four militants in the northwestern tribal area of Pakistan, which is awash with Taliban and Al-Qaeda-linked networks and where US special forces killed Osama bin Laden on May 2.
More than 500 people had packed into the mosque and a senior official from the Khyber tribal district administration Sayed Ahmed Jan told AFP that the bomb had exploded seconds after the main prayer ended.
"The United States deeply respects Pakistan's sacrifices in the fight against extremism and we continue to stand with Pakistan against those who seek to undermine democracy," said Clinton.