Updated

The United States is pouring $83 million into upgrading its main military bases in Afghanistan, an Air Force general said Monday in a sign that American forces will likely be needed in the country for years to come as Al Qaeda (search) remains active in the region.

Meanwhile, in a reminder of the instability still facing the 25,000 foreign troops in the country, a roadside bomb hit a Canadian Embassy vehicle and another car in Kabul (search), injuring at least four people.

U.S. Brig. Gen. Jim Hunt said the millions were being spent on construction projects already under way at Bagram (search), the main U.S. base north of Kabul, and Kandahar in the south. Both are being equipped with new runways.

"We are continuously improving runways, taxiways, navigation aids, airfield lighting, billeting and other facilities to support our demanding mission," Hunt, the commander of U.S. air operations in Afghanistan, said at a news conference in the capital.

Afghan leaders are seeking a long-term "strategic partnership" with the United States, which expects to complete the training of the country's new 70,000-strong army next year, but it remains unclear if that will include permanent American bases.

Secretary of State Condoleezza Rice (search) said in Kabul this month that Washington had not decided how long to keep troops in the country, which neighbors Iran, Pakistan and oil-rich Central Asia.

U.S. commanders have said they may cut their 17,000-strong force this year if a Taliban insurgency wanes. But they say the Afghan government remains vulnerable and some kind of U.S. presence will be needed for years.

In a televised interview, Army Gen. John Abizaid (search), the commander of U.S. Central Command, said fresh skirmishes along the Pakistani frontier showed "the fight is not out of the Taliban completely, and not out of the Al Qaeda people that are operating in that region."

Asked where Usama bin Laden might be, Abizaid said only that "an awful lot of Al Qaeda leadership" was operating in the mountainous border region and that U.S. troops were watching the area "with great interest."

Hunt said 150 U.S. aircraft, including ground-attack jets and helicopter gunships as well as transport and reconnaissance planes, were using 14 airfields around Afghanistan. Many are close to the Pakistani border. Other planes such as B-1 bombers patrol over Afghanistan without landing.

"We will continue to carry out the ... mission for as long as necessary to secure a free and democratic society for the people of Afghanistan," Hunt said.

American officials say fixing the runway at Bagram will make it suitable for Dutch F-16 fighters expected to deploy this year to support the separate NATO-run security force in Afghanistan. U.S. forces have begun vacating Shindand Air Base, near the Iranian border, as NATO expands its 8,500-strong force into the west.

U.S. forces are increasingly focused on the area along the Pakistani border, but security remains fragile also in the capital.

Monday's explosion damaged a Canadian Embassy vehicle and injured one Canadian, Afghan officials said. The bomb left a five-foot-wide crater next to the road. However, witnesses said the man, identified by an embassy official as a security guard, walked unaided from the damaged vehicle.

Canadian officials were investigating the blast.

"It detonated just as the embassy vehicle was driving by," Lt. Col. Roland Lavoie, a Public Affairs spokesman for Canada's Department of Defense, said from Ottawa.

"One of the goals of the investigation is to determine whether Canadian vehicles were targeted. At this stage we don't know," Lavoie added.

Three Afghan men traveling in another car caught in the blast were taken to a hospital, one of them for serious injuries, Wazir Gul, the driver, said.