Suspected militants in southern Pakistan Thursday torched three oil tankers carrying oil for U.S.- led NATO forces in neighboring Afghanistan, local media and witnesses said.

The gunmen, riding motorcycles, first opened firing on the NATO oil tankers in district Shakarpur in Sindh province, and torched three tankers, witnesses said.

They said the unidentified armed men first opened fire and then torched the NATO oil tankers on the main highway, linking Karachi with the northwest, the main supply route to Afghanistan.

Traffic was also suspended on National Highway for a while and police later arrived at the scene and also searched the area to apprehend the attackers but no one was arrested.

The oil tankers were coming from the port city of Karachi and heading to Afghanistan when came under attack. No one was injured.

The gunmen fled after the attack while firing in the air to keep the people away from them.

Fire fighters were called out to extinguish the blaze. No group claimed responsibility for the attack. The government blames militants for attacks on NATO supply trucks and oil tankers.

Attacks on NATO vehicles in the country's insurgency-hit northwest and southwest are common and suck attacks in southern Sindh province is rare.

It is second attack on NATO trucks in Pakistan in two days.

On Tuesday, two people were killed when gunmen torched an oil tanker carrying fuel for NATO troops in Afghanistan in southwestern Baluchistan province.

Hundreds of tankers and trucks daily cross into Afghanistan from Pakistan to carry oil and other items for some 150,000 U.S.- led NATO forces there.

Pakistan is the shortest supply route to Afghanistan and officials said that around 70 percent items are supplied to NATO forces via Pakistan.

The U.S. has struck deals with Russia and some Central Asian states for alternate supply routes in view of the attacks on NATO trucks in Pakistan.