The Afghan government, meanwhile, launched an urgent appeal for $76.4 million to tackle an "imminent food crisis" caused by prolonged drought, particularly in the north and northwest of the country.
The bomb in Kabul, the latest in a series of recent blasts that have rattled nerves in the capital, killed a man and a woman riding in taxi, and wounded four others, said Faiz Ahmad Hotaq, a police official.
In the eastern Kunar province, a U.S. soldier was killed Monday in firefight with militants, said Col. Tom Collins, a coalition spokesman.
Seven militants were also killed Monday in Paktika province in clashes with coalition soldiers, it said. One coalition soldier was slightly wounded.
Violence has escalated sharply in Afghanistan this year, as Taliban-led rebels have stepped up attacks, particularly in their former southern heartland, drawing a tough response from Afghan and foreign forces.
More than 600 suspected Taliban militants have been killed since a U.S.-led offensive began last month in southern Afghanistan, a coalition spokesman said Tuesday.
Collins said the 600 militants have died in combat since Operation Mountain Thrust started June 10. The offensive is aimed at crushing the deadliest spate of Taliban violence since the hard-line regime's 2001 ouster.
At least 19 coalition soldiers have also been killed in southern Afghanistan in the same period, according to according to an Associated Press count based on coalition information.
More than 10,000 U.S.-led troops have been operating in former Taliban strongholds across southern Afghanistan. The region has witnessed the brunt of the deadliest upsurge in Taliban-led violence since the hard-line regime's 2001 ouster.
NATO-led forces are preparing to take over command of security operations there, a move that could lead to a reduction in the more than 21,000 U.S. troops in Afghanistan.
The U.S. military said Tuesday that two American engineer soldiers were seriously wounded in a roadside bomb attack in eastern Khost province.
The two were on their way to a road project Sunday between the towns of Khost and Gardez when they were attacked, the military said. Their wounds were serious but not life threatening.
President Hamid Karzai, meanwhile, condemned the fatal shooting of an Afghan doctor and a driver for the international Christian relief and development organization World Vision on Sunday. The pair were killed after they had delivered medicines to the town of Charsada in Ghor province, a rare attack in a relatively stable region.
Karzai said in a statement that the two were killed "at the instructions of foreigners," but did not elaborate.
Afghan officials often accuse eastern neighbor Pakistan of providing a haven for Taliban-led guerrillas who have stepped up their insurgency this year, often targeting government officials and aid workers. Pakistan denies it, saying it does all it can to prevent cross-border infiltration by militants.
As violence grips the country, the Afghan government and United Nations launched an emergency appeal Tuesday for 76.4 million to tackle an "imminent food crisis" in Afghanistan because of drought.
That money would support the "urgent needs" of more than 2.5 million affected people for six months, a joint statement by Afghan government and the U.N. said.
Afghanistan is facing a shortfall in this year's rain-fed wheat harvest because of a prolonged drought, particularly in the north and northwest of the country.
Agriculture accounts for 52 percent of the impoverished nation's gross domestic product and wheat comprises 80 percent of cereal production.