Imam said the identities of the victims were not yet known.
He said police were not aware that the depot was used by vehicles carrying supplies for Nato and US forces in Afghanistan.
"The attack took place around 11:35pm [18:35 GMT] and we are still trying to find out how this attack has happened," Shah Nawaz, head of Tarnol police station, said.
Much of the supplies and fuel for the US-led force in landlocked Afghanistan are transported in lorries through Pakistan after arriving on ships that dock at the Arabian Sea port of Karachi.
Thousands of lorries make the journey across the border each week, making it a lucrative business for transporters.
Imtiaz Gul, a security analyst in Pakistan, told Al Jazeera that the raid was probably carried out by Pakistani Taliban fighters who have made a habit of targeting Nato convoys.
"Until a few months ago, Afghanistan-bound vehicles had been transiting through the city of Peshawar," Gul said.
"They still do but following a couple of hundred attacks and the torching of several hundred containers, the transporters decided to relocate the parking and overnight stops closer to Islamabad.
"We are told the terminals they use for parking didn't have enough security. It would be very difficult and expensive for the US to find a different route to Afghanistan."
Fighters with bases along the remote Afghan border in Pakistan's northwest regularly attack government and security force targets.
Also on Wednesday, Pakistani officials said fighters attacked two security checkpoints in Mohmand, in the northwest tribal belt.
The overnight attack sparked clashes that killed two soldiers and six fighters and wounded several from both sides.
On Tuesday night in Orakzai, near the border with Afghanistan, fighters attacked a security convoy leading to a battle that killed six soldiers and 40 fighters, a Pakistani official said on Wednesday.
Fifteen soldiers also were wounded in the fighting, Samiullah Khan, a government official, said.