KABUL - A suicide bomber rammed a car into a convoy of NATO forces close to the airport in the Afghan capital on Tuesday, wounding 10 Afghan civilians, a police official said.

A spokesman for the Taliban said the militant Islamic group carried out the attack to "welcome" U.S. Defence Secretary Robert Gates who arrived in Kabul on Monday evening.

There were no casualties among NATO troops in the blast which happened during the morning rush hour on a road just outside Kabul's international airport, an alliance spokesman said. NATO put the number of Afghans wounded at 22.

The spokesman for NATO, which runs a force separate to the U.S. military in Afghanistan, said that one of its vehicles was damaged in the blast.

Foreign and Afghan troops sealed off the site of the attack and smoke could be seen rising from the area.

Tuesday's blast reflected a rising level of violence in the past two years in Afghanistan, the bloodiest period since the Taliban's removal from power in 2001, that has left an increasing number of Afghans frustrated with the lack of progress.

A poll conducted for three Western television networks showed that only 42 percent of Afghans rated U.S. efforts positively, down from 68 percent in 2005 and 57 percent last year.

The poll conducted for ABC News of the United States, BBC and Germany's ARD network also found that support for the Taliban fighters is rising. Twenty-three percent of those polled in the troubled southwest said people in their area support the Taliban, triple last's year level.

The United States has 26,000 troops in Afghanistan. About half are involved in NATO operations and half on other missions.

Gates plans to gauge the needs of military commanders as they face a resurgent Taliban and signs that al Qaeda is regrouping.

More than six years after a U.S.-led invasion drove the Taliban from power, Gates said on Monday he was concerned about the increasing violence but he did not think Afghanistan was moving backwards.

"I'm not worried about a back slide as much as I am how we continue the momentum going forward," the Pentagon chief told reporters. "I think that one of the clear concerns that we all have is that (in) the last two or three years there has been a continuing increase in the overall level of violence."

The Pentagon also is worried about signs that al Qaeda is resurfacing in Afghanistan after losing ground in parts of Iraq.