A Taliban homicide bomber attacked a NATO convoy outside Kabul, killing at least eight people Tuesday, hours after mortar rounds struck near the presidential palace, as militants made good on their promise to sow mayhem in the run-up to Afghanistan's presidential election.

Two U.S. soldiers were killed and three wounded in a separate blast in the country's east, the U.S. command said.

Despite the uptick in violence, NATO announced Tuesday that its forces would refrain from offensive military operations on election day and would undertake missions only if they were "deemed necessary to protect the population."

Two Afghans working for the United Nations were among the dead and another was wounded in the homicide attack, the U.N. office in Kabul announced. One U.N. staffer was among the 50 people that the Interior Ministry said were wounded.

The homicide bomber used a car to strike the convoy as it traveled along a road near a British military base in the eastern edge of the city. The ministry said seven people were killed. NATO said one of its troops was also killed and two others were wounded.

Zabiullah Mujahid, a Taliban spokesman, claimed responsibility for the blast in a telephone conversation with The Associated Press. The group has denounced the election and warned people to stay away from the polls.

British troops were guarding the site of the homicide attack as rescuers rushed the wounded to hospitals. An AP reporter at the scene saw British soldiers collecting what appeared to be body parts from the roof of an Afghan home. He also reported shouting matches between the British troops and Afghan security personnel at the blast site.

About a dozen private vehicles were destroyed near the road where the attack happened. People used their hands to dig through the rubble of damaged buildings. Families carried the wounded away from the scene.

U.S., NATO and Afghan security forces are on high alert this week because of the Thursday vote. President Hamid Karzai is favored to win but faces a stiff challenge from former Foreign Minister Abdullah Abdullah. About three dozen candidates are in the race.

The attack against U.S. troops occurred in eastern Afghanistan, the U.S. military said in a statement. It said two were killed and three were wounded when their vehicle struck a bomb but gave no further details.

Those deaths brought to 26 the number of U.S. service members killed in Afghanistan this month, according to an AP count.

Elsewhere, a homicide bomber struck the gates of an Afghan army base in the southern province of Uruzgan, killing three Afghan soldiers and two civilians, provincial police chief Juma Gul Himat said.

U.S. military spokeswoman Capt. Elizabeth Mathias had no details of damage or casualties from the attack on the presidential compound.

Neither Karzai or anyone else was wounded in the attack, said deputy presidential spokesman Hamid Elmi. He said the rounds probably hit "somewhere around the compound," but he had no further details.

Attacks in Afghanistan have risen steadily the last three years. In a speech Monday in Phoenix, President Barack Obama said U.S. troops would help secure polling places so that the elections can go forward and Afghans can choose their own future.

Obama said peace in Afghanistan "will not be quick" and "will not be easy." He added that the United States still has a deep interest in the long-term outcome.

"This is not only a war worth fighting. This is fundamental to the defense of our people," Obama said.