A homicide bomber on Monday killed the Pakistan army's surgeon general, the highest-ranking military officer to die in an attack since President Pervez Musharraf joined America's war against terror.

The strike was a reminder of the extremist threat just as Musharraf resists pressure to quit from opponents who swept last week's parliamentary election.

A presidential spokesman on Monday dismissed suggestions from three U.S. senators that the embattled Pakistani leader might beat a "graceful" retreat from power.

Musharraf was elected to a new five-year presidential term last year by Pakistani lawmakers, "not by any senator from the United States," spokesman Rashid Qureshi told Dawn News television. "So I don't think he needs to respond to anything that is said by these people."

The lone homicide bomber targeted Lt. Gen. Mushtaq Baig's black Toyota sedan when it stopped for a red signal on a busy road in Rawalpindi, a city just south of the capital, officials and witnesses said.

Baig, the army's top medical officer, died along with his driver and guard, the army said. Five civilians were also killed, it said.

Dozens of troops and plainclothes security officials cordoned off the area, where at least six other damaged cars stood. Plastic-gloved investigators gathered debris, including pieces of flesh, and put them in plastic bags.

Iqbal Ali, a witness who had been walking toward a nearby government office to get an identity card, said he saw a man run into the road shortly before the explosion.

Army spokesman Maj. Gen. Athar Abbas said Baig was the most senior army officer killed in an attack since Pakistan sided with Washington after the Sept. 11, 2001 terrorist attacks in the United States.

Musharraf himself survived at least three attempts on his life before he retired as army chief in November.

Abbas said it was unclear who was behind Monday's attack.

The government blamed "extremist elements" who were "damaging the cause of Islam" and spoiling Pakistan's international reputation. Authorities will "not let the perpetrators succeed in their evil designs," caretaker Prime Minister Mohammedmian Soomro said.

Homicide bombers have struck repeatedly in Rawalpindi, where the army has its headquarters, in recent months, mostly targeting security forces. A gun and homicide attack also killed opposition leader Benazir Bhutto as she left a campaign rally in the city on Dec. 27.

Bhutto's party finished first in Feb. 18's parliamentary elections, while supporters of Musharraf were trounced, prompting some opposition leaders to call for him to resign.

The Bush administration appears to want Musharraf to continue in office even as it shifts its focus to his successor as army chief and the next government.

However, Sen. Joe Biden, chairman of the Senate Foreign Relations Committee, who met Musharraf after observing the election, said Sunday he would advise the president to seek a dignified way to leave office.

"I firmly believe if they (political parties) do not focus on old grudges — and there's plenty in Pakistan — and give him a graceful way to move," then it could happen, Biden, a Democrat, said on ABC television.

Republican senators Kay Bailey Hutchison and Chuck Hagel also endorsed a negotiated Musharraf departure. They stopped short of saying Musharraf should be pushed from power.

The parties of Bhutto and another former premier, Nawaz Sharif, are expected to form a coalition government.

Still, they fell short of the two-thirds majority needed to impeach Musharraf, whose popularity plummeted last year after he declared a state of emergency and clamped down on the judiciary and the media.

Sharif, whose second government fell when Musharraf seized power in a 1999 military coup, on Monday urged Musharraf to convene the new parliament.

"The transfer of power should happen immediately," Sharif told reporters in Islamabad. "The sooner Mr. Musharraf understands the situation, accepts people's verdict and resigns, the better it is for him."

Bhutto's party has been more cautious.

Asif Ali Zardari, Bhutto's widower and political successor, said in remarks published Monday that his priority was for the transition to remain smooth.

"We want to unify the country, which is facing some very serious challenges," Zardari was quoted as saying in The Wall Street Journal. "We have to establish democracy and for that we need unity and not confrontation."

Qureshi said the president wanted to cooperate with the incoming lawmakers.

"The president is an easy man to get along with and I don't think we should feel that there'll be any friction there."

Western officials are concerned that an attempt to force Musharraf from power would spark a constitutional crisis and hobble Pakistan's efforts against Islamic extremism.

Also on Monday, gunmen opened fire and threw grenades inside the office of the British-based aid group Plan International in the northern town of Mansehra, killing four local staff, the group and police said.

The identity and motive of the attackers was unclear.