Police on Friday said they had arrested 10 suspects after the attack, which was carried out by at least a dozen gunmen. The tankers had been parked near a filling station on a highway leading north from Karachi.

Apparently, nobody died in the assault. It was the first major attack on NATO trucks in southern Sindh province, which is not known as a hotbed of Islamist militancy. Until now, convoys have mostly come under attack in the country's northwest.

Islamabad angry about border violations

Meanwhile, the main route used by NATO convoys to carry supplies into Afghanistan remained blocked on Friday, according to Pakistani officials.

The Pakistani authorities shut the Torkham border for NATO trucks on Thursday in protest against several incursions into Pakistani airspace by NATO helicopters from Afghanistan this week. In the latest such incident, three Pakistani border troops were killed by NATO forces that said they were in "hot pursuit" of militants.

The border closure has highlighted the leverage that Pakistan has over Washington in this issue. About three-fourths of NATO supplies are carried into Afghanistan through Pakistan, most of them via the Torkham border near the Khyber Pass. Another less used route crosses the border at Chaman in Balochistan.

But experts point out that Islamabad cannot afford to alienate Washington too much, as it is highly dependent on American aid. The US special envoy to Afghanistan and Pakistan, Richard Holbrooke, expressed optimism on Friday that the border closure would be lifted soon.

tb/AFP/Reuters/dpa/AP

Editor: Anne Thomas