At least five people were killed as 40 NATO oil tankers were attacked early Friday morning by some unknown militants in Pakistan's northwest city of Peshawar, reported local English TV channel Express.

According to the report, the incident took place at about 2:00 am (local time) Friday when some unknown militants fired at the NATO oil tankers parked by the roadside in the downtown area of Peshawar, capital of Khyber Pakhtunkhwa province in northwest Pakistan.

The attack caused a huge fire on the oil tankers. Fire tenders rushed to the site immediately following the report of the attack. It took hours to bring the fire under control, the report quoted an eyewitness as saying.

Four people, believed to be the drivers and cleaners of the containers, died during the attack, said the report, adding that the police have sealed off the site and launched an investigation into the case.

No group has claimed responsibility for the attack yet.

Local watchers believe that the attack could be a retaliation against the recently stepped-up strikes by the U.S. drones in Pakistan. Over the last five days, the U.S. drones have launched two drone strikes in Pakistan's South Waziristan and North Waziristan, both of which are believed to be strongholds of militants in the country's northwest tribal area bordering Afghanistan, killing at least 19 people altogether.