BAGHDAD, Iraq - A mortar barrage hammered the heavily fortified Green Zone and elsewhere in central Baghdad on today, killing at least one person and underscoring the vulnerability of even Iraq's best-protected areas ahead of national elections.

Also today, a car bomb exploded next to a Bradley fighting vehicle near Beiji, about 155 miles north of Baghdad, wounding two U.S. soldiers and two Iraqi National Guardsmen, said Master Sgt. Cynthia Weasner of the 1st Infantry Division. No other details were available.

The Pentagon said Wednesday that to increase security, U.S. troop strength in Iraq will be raised from 138,000 now to about 150,000 by mid-January - the highest level of the war. It said extra troops are needed to secure Iraq's Jan. 30 election.

In Baghdad, at least five mortar rounds exploded, including two in Baghdad's Green Zone, the compound that holds Iraq's interim administration and foreign diplomatic missions.

One round struck near a mobile phone office in Baghdad's Arasat neighborhood, killing one person and wounding three, according to police Lt. Muhsin Khazim. Witnesses also reported people were wounded in the other blasts around Baghdad, but the number of casualties wasn't known.

Last week, a mortar attack killed four Nepalese employees of a British security firm and wounded at least 12 in the Green Zone.

In the northern city of Mosul, 10 insurgents were killed in a clash with Iraqi security forces, an Iraqi general said today.

Maj. Gen. Rasheed Feleih said two rebels were captured during the firefight, which flared overnight when a police special forces unit was conducting a sweep through the city's troubled Islah neighborhood.

Mosul's 5,000-member police force disintegrated during an insurgent uprising last month, forcing the U.S. command and the interim government to divert troops from the offensive in Fallujah.

And in Baladruz, a town, about 50 miles northeast of Baghdad, attackers gunned down a police colonel as he walked through a market, according to a policeman there, Ahmed Hassan. Insurgents have repeatedly targeted Iraqi police, who are seen as cooperating with U.S. forces.

Also today, the U.S. Embassy banned its employees from using the highway linking Baghdad to the international airport, regarded as one of the country's most dangerous roads.

"Until further notice, no mission personnel will be permitted to use the main road from the international (Green) zone to the Baghdad International Airport for security reasons," a warden's message said.

The 10-mile stretch of road has been the scene of repeated roadside bomb and suicide attacks generally targeting U.S. military personnel and civilian contractors working on reconstruction projects in Iraq.

Insurgents have steadily stepped up attacks in Iraq, particularly in Baghdad, ahead of the scheduled elections.

The embassy statement also cautioned U.S. citizens in Iraq to review their security and warned those planning to travel to Iraq whether such a trip was "absolutely necessary."

On Monday, the British Foreign Office issued a similar warning ordering its staff from traveling on the airport road for security reasons.