Insurgents killed an American soldier in the restive city of Mosul (search) and mortar strikes pummeled central Baghdad on Thursday, while the U.S. Embassy barred employees from the dangerous highway leading to the airport.

Despite the violence, a top Iraqi official insisted the security situation had improved since U.S. forces scattered insurgents in the Sunni Muslim city of Fallujah (search) last month in preparation for elections set for Jan. 30.

To provide security for the election, the U.S. government has announced it is raising troop strength in Iraq to its highest level of the war. The number of troops will climb from 138,000 now to about 150,000 by mid-January — more than in the 2003 invasion.

While Iraq's Kurds and majority Shiites back the elections, Sunni groups have demanded a postponement because of the poor security. President Bush dismissed those calls Thursday, insisting the elections must not be delayed.

"It's time for Iraqi citizens to go to the polls," Bush told reporters in the Oval Office.

The American soldier who was killed Thursday was on a U.S.-Iraqi patrol in Mosul when the attack occurred, Lt. Col. Paul Hastings said. He said two Iraqi commandos were also wounded.

Hastings also said Iraqi and U.S. forces discovered 14 unidentified bodies in Mosul on Thursday. He said there were also reports of five more bodies picked up by family members. That brings to at least 66 the number of bodies — many of them believed members of the Iraqi security forces — found there since Nov. 18.

Mosul's police force disintegrated during an insurgent uprising last month, forcing the U.S. command to divert troops from the offensive in Fallujah.

Also Thursday, attackers launched at least five mortars in central Baghdad, including two that crashed into the Green Zone (search), the compound that houses Iraq's interim administration and U.S. diplomatic missions.

One round struck near a mobile phone office in the Arasat neighborhood, killing two civilians and wounding five, U.S. military spokesman Lt. Col. James Hutton said.

U.S. forces have been helpless to stop insurgents from firing mortars into the heavily fortified area, a sharp reminder of how efforts to calm to the country ahead of the elections have been frustrated.

U.S. senators visiting Iraq on Thursday said they were pleased with Bush's decision raising troop levels, but criticized him for not doing so earlier.

"We should have leveled with the American people in the beginning," Sen. Joseph Biden, a Democrat from Delaware, told reporters. "It was absolutely inevitable" that more troops would be needed, he said.

A symbol of the Americans' struggle to restore security has been the airport highway. Attackers using car bombs and explosives have stymied U.S. efforts to protect the road, one of the country's most crucial yet also one of its most perilous.

The U.S. Embassy decision to ban its employees from using the highway followed a nearly identical warning Monday from Britain's Foreign Office. The embassy also cautioned Americans in Iraq to review their security situation and warned those planning to travel to Iraq to consider whether the trip was "absolutely necessary."

However, Qassim Dawoud, Iraq's national security adviser, said insurgent attacks were down since the invasion of Fallujah. He provided no details but said Iraq didn't need U.S.-led coalition forces' help to safeguard the election.

"We don't want to involve the multinational forces in the election affairs," he said. "We are taking our measures to provide security to our society, specifically to the electoral centers."

Defense Minister Hazem Shaalan told Al-Jazeera television that Iraqi tanks and armored vehicles would patrol the streets during the election to ensure security.

On Thursday, an official of a Sunni political party said the group would push forward with demands to postpone the elections. The Iraqi Islamic party and other advocates of a postponement will hold a conference Sunday in a bid to muster more support for a delay, said party official Ammar Wajeeh.

"We are not convinced that elections could be held on this date so long as the security situation remains the way it is," Wajeeh said.

Last week, influential Sunni politicians urged the government to postpone the voting for six months to give authorities time to secure polling stations and to persuade Sunni clerics to abandon their call for an electoral boycott.

Also Thursday:

— A car bomb in the city of Beiji, about 155 miles north of Baghdad, wounded two U.S. soldiers and two Iraqi National Guardsmen, the U.S. military said.

— The U.S. military said a vehicle accident in western Baghdad on Wednesday killed one U.S. soldier and injured four.

— Two Iraqi National Guardsmen were killed when a roadside bomb exploded near their car in Baghdadi, about 140 miles west of Baghdad, an Iraqi National Guard official said.