In the first incident, US soldiers were patrolling in the restive southeastern province of Paktika ahead of parliamentary elections next month when a roadside bomb hit their armoured vehicle, the US military said in a statement.
"One US service member was killed and four were wounded when an improvised explosive device struck their up-armored high-mobility wheeled vehicle near Khayr Kot district, Paktika province on Friday," it said.
The incident comes less than a week after four US soldiers were killed and three wounded in a similar attack in the southern province of Zabul.
"The four US service members wounded in the attack were evacuated to nearby bases for treatment where one is in critical condition, one in stable condition and two have been returned to duty," the US military statement said.
The soldiers were part of a combat patrol targeting rebels who have threatened to disrupt the upcoming elections on September 18, it added.
An Afghan woman holds a campaign
leaflet in Kabul for the coming polls
"Our forces have gained a great deal against the enemy and our actions will ensure a safe and secure election next month," said Brigadier General James G. Champion, deputy commanding general of US operations in the country.
In a separate attack, three US soldiers were wounded when their convoy came under attack about 40km east of Kabul on Friday.
"Coalition forces were conducting operations to disrupt enemy forces when they came under fire. A US Army attack helicopter responded to the scene but the enemy had fled the area," the military said in a separate statement Saturday.
The wounded were being treated at Bagram Airbase coalition headquarters north of Kabul and were in stable condition.
"The doctors are doing all they can to make sure each of them makes a swift and full recovery," Lieutenant Colonel Michael Doyle said in the statement.
The latest casualties bring to 75 the number of soldiers killed this year in operations linked to Afghanistan.
This has been the bloodiest year in Afghanistan since the Taliban were driven from power by US-led forces in 2001, with almost 1000 people killed so far in political violence compared with 850 four years ago.
Missing Japanese teachers
Afghan authorities said on Saturday that they have launched a massive search for two Japanese schoolteachers missing in the region, while a Japanese diplomat said he was optimistic they are still alive and have not been kidnapped.
Two Taliban commanders in southern Afghanistan said the rebels had not abducted or killed the pair.
The two teachers, a man and a woman, crossed into Afghanistan from neighboring Pakistan on 8 August and have not been seen or heard from since, said Kenji Saito, the first secretary at the Japanese Embassy in Kabul.
The two had been expected to return to Japan last week but have not come home.
"If they had been killed, someone would have found the bodies and we would know. I don't believe they are dead," Saito said in an interview with The Associated Press. "We also don't believe that they have been kidnapped, because no one has contacted us to make demands.
"But we have no information as to where they are, though we are doing our best to find out," he said.
Entry in question
Afghan Interior Ministry spokesman Latfullah Mashal said police in areas near the Pakistan border have been ordered to search for the pair, but he said it was still not clear whether they actually entered Afghanistan.
He said immigration officials had no record of two Japanese crossing into the country by road from Pakistan during the past 30 days. He said they may have entered the country through an unofficial border crossing, or they may be lost in lawless tribal areas on the Pakistani side of the frontier.
Taliban commanders Mullah Mohammed Nazir and Mullah Mohammed Alum, both of whom are in different parts of southern Afghanistan, told The Associated Press by satellite telephone that they had not kidnapped or killed the two.
"If we had kidnapped them or killed them, we would claim responsibility," Alum said.
The two junior high school teachers went to Pakistan on 6 August and were to return to Japan on the 19th, said Akito Makihara, an education board official for Hiroshima prefecture (state) in western Japan. They apparently came to Pakistan and Afghanistan for tourism.
In the Pakistani city of Quetta, where they stayed, hotel manager Ain ud-dinm identified the pair as Jun Fukusho and Shinobu Hasegawa.