ISLAMABAD, Pakistan - A man blew himself up yesterday outside a major hotel used by government officials and foreign dignitaries, killing himself and a security guard, police and security officials said. Three people were wounded.

No one immediately claimed responsibility for the attack; the government put security forces on high alert across the country to guard against any follow-up attacks.

The bomb went off at 2:40 p.m. in an affluent section of the Pakistani capital, and seemed to demonstrate that extremists are able to readily mount attacks in what have been regarded as zones of high security.

Foreign visitors regularly stay at the hotel, the Marriott, and it is often the site of government and business conferences and functions. The residences of the prime minister and several cabinet ministers are nearby; so are the parliament complex and a number of embassies.

The U.S. Embassy issued a caution to American citizens in Pakistan, saying it "strongly advises all Americans to avoid the area, exercise caution, and limit unnecessary travel."

Government officials and witnesses said the suicide bomber, apparently a man in his 20s, tried to enter the hotel through a side entrance near the parking area.

"There is an emergency gate which is used by the employees of the hotel," said Javed Iqbal Cheema, a retired army brigadier who heads the national crisis-management office, in an interview at the site of the blast. "He wanted to make a forced entry into the hotel for himself. The guard, as a matter of fact, resisted entry into the gate and did not allow him to get inside. On seeing that he can't make his entry, he blew himself up."

Cheema said the police had not yet established how the bomber traveled to the hotel, saying only that he was on foot as he approached the entrance.

Police officials said the bomber's head and upper body were obliterated in the explosion, leaving little intact for forensic scientists to examine other than parts of lower limbs.

The blast also shattered the windows of several vehicles parked nearby.

No guests of the hotel were harmed, according to a statement by the district magistrate's office. But three hotel employees were injured and were sent to the Poly Clinic hospital for treatment.

Pakistani officials said it was too early to say who might have planned the attack. Local extremists and al-Qaeda-related groups have assailed President Pervez Musharraf since he allied the country with the United States in fighting terrorism after the Sept. 11 attacks. But multinational chains such as Marriott and Sheraton hotels and KFC and McDonald's restaurants have also been frequent targets.

Syed Kamal Shah, the interior secretary, said there was no sectarian motive to the attack, and said it was a simple act of terrorism.

"It is certainly not aimed at creating disturbance on the eve of Ashura," Shah told the state-run news agency.

Ashura is the preeminent religious event of the year for Shiite Muslims, commemorating the death of one of the sect's founding martyrs, Muhammad's grandson, Imam Hussein. Tensions between extremist Shiites and extremist Sunnis often run high around Ashura, which falls this year on Monday and Tuesday.