ISLAMABAD, Pakistan - A huge truck bomb exploded outside a luxury hotel in northwestern Pakistan's provincial capital last night, killing 11 people and injuring at least 50, officials said. The attack marked the latest salvo by insurgents who have vowed to avenge an army offensive in the nearby Swat Valley, and it underscored their ability to strike at some of the country's most heavily fortified targets.

Peshawar's Pearl Continental Hotel had been considered an oasis of relative security in a city that has become a front line in the battle between the Pakistani government and groups such as al-Qaeda and the Taliban.

The hotel had been popular among foreigners, including aid workers who have been using it in recent weeks as a base for their efforts to assist the more than two million Pakistanis displaced from their homes by the fighting in Swat.

The bomb, hidden beneath kitchen supplies and estimated to contain more than 1,000 pounds of explosives, caused a section of the hotel to collapse and destroyed more than 30 vehicles, said Shafqatullah Malik, an assistant police chief.

The dead included at least two foreigners, according to law enforcement and hospital sources. One was an official with the office of the U.N. High Commissioner for Refugees, a U.N. spokesman said. He had been part of a group of U.N. staff that had been working to assist displaced families.

U.S. Embassy spokesman Lou Fintor said there was no indication that any Americans had been hurt or killed. U.S. government personnel had been prohibited from the hotel because of security concerns. Last week, the embassy issued a warning to Americans to avoid Peshawar altogether. The city has come under increasing strain as the Pakistani Taliban has strengthened, and bombings there have become a regular occurrence.

The Pearl Continental, part of a chain of five-star Pakistani hotels, is in one of the most heavily fortified areas of the city, set near a cluster of government buildings, including a courthouse, the provincial chief minister's house, and a major military base.

The State Department has been negotiating to buy the hotel and convert it into a consular building in Peshawar, which is the capital of North-West Frontier Province and the largest city in the region. That deal has not been completed.

Pakistani television networks broadcast images of dazed hotel guests and workers evacuating the building, many covered in dust and some with bloodied faces. The blast demolished the hotel's kitchen, and security guards and kitchen staff appeared to have constituted a large share of the injured, according to hospital officials.

Police and an intelligence official at the scene reported that two vehicles were involved in the attack. First, a Toyota Corolla drove up and distracted the guards. Then a mini-truck followed. The truck's passengers opened fire on the guards at the gate as the truck sped toward the building. Moments later, the blast shook the hotel and reverberated across the city. At least 50 rooms were damaged, and 10 were destroyed.

President Asif Ali Zardari condemned the attack, saying: "Such incidents will not deter the government from its resolve to eliminate this scourge from the country."

Two hotel security guards were taken into custody for questioning, officials said.