KABUL (AFP) – U.S.-led and Afghan troops continued their push against the Taliban on Friday after killing 40 insurgents in an air and ground strike, while NATO warned it would deal "robustly" with rebels.

The "enemy fighters" were killed in a strike in a remote, mountainous rebel stronghold in southeastern Paktika province, on the border with Pakistan, the U.S.-led coalition force said late on Thursday.

The strike -- which lasted throughout Wednesday and Thursday -- was part of Operation Mountain Thrust, the biggest push against militants since the fall of the Taliban government nearly five years ago, the coalition said in a statement.

Involving more than 10,000 Afghan and coalition troops, who have been deployed in the south over the past few months, the operation was launched in mid-May.

It is intended to "set the conditions" for NATO's takeover in the coming weeks of command of the volatile south, which is the birthplace of the Taliban movement and sees the most insurgency-linked violence.

Despite the presence of more than 30,000 troops and the growing capacity of the Afghan security forces, insurgents have been able to launch significant attacks in the past weeks, increasing fears of a Taliban comeback.

The rebel movement, which wants foreign "infidels" to leave Afghanistan, directs most of its attacks at Afghan and coalition troops. But it also targets people siding with the new government or working on reconstruction projects.

On Thursday 13 Afghans were killed in three different bomb blasts, apparently aimed at people working with foreign organizations.

Ten people were killed in the deadliest attack on civilians in months when a time bomb exploded on a bus taking workers to a coalition-run air field in the southern city of Kandahar.

Five interpreters, a cleaner and a driver were killed on the bus, coalition spokesman Major Quentin Innes said. The three other dead were bystanders. Sixteen people were wounded, he said.

Taliban spokesman Yousuf Ahmadi said the group was targeted because "these people were working for Americans".

"Anyone, anywhere in Afghanistan working for U.S. forces will be targeted. They must quit working for Americans or their friends," he told AFP by telephone from a secret location.

In a separate attack two men employed by a Turkish road construction company were killed when a bomb exploded under their vehicle in southwestern Nimroz province.

An Afghan man contracted to the United Nations demining agency was killed in another bomb blast on a vehicle in the same province, a spokeswoman for the organization said.

NATO's International Security Assistance Force (ISAF) is due in late July to take over security operations in the south from the U.S.-led coalition, assuming command of thousands of British, Canadian, Dutch and U.S. soldiers as well as smaller contingents from other countries.

The military alliance's secretary general, Jaap de Hoop Scheffer, reaffirmed in Canada on Thursday that the mission's aim was to prevent Afghanistan from reverting to a state exporter of terrorism.

"The Taliban are simply testing Canadian public opinion, Dutch public opinion, UK public opinion, Australian public opinion," he told a news conference.

But he added that NATO would know how to deal with those who want to stop the process of reconstruction in the war-torn country.

"The message to the spoilers -- be it Taliban, be it drug lords or warlords, whatever -- will be a very stern and strong message: 'You will not get it your way. You will be dealt with very robustly if necessary'," he said.