On Sunday, a US soldier was killed and another hurt in an attack in the southern province of Zabul and two US soldiers were wounded on Monday in the eastern province of Kunar, the US military said.
Officials said one bomber blew himself up in the centre of Kandahar on Monday, killing a senior anti-Taliban commander and three others.
Eight people were also hurt in the blast, which was followed by a second in which a bomber died on the road leading to the airport when a bomb strapped to his body exploded prematurely, said Kandahar Governor Assadullah Khalid.
There were no other casualties, he said. US-led foreign troops are based at Kandahar airport.
"Both incidents were the work of suicide bombers of Taliban and al-Qaida," Khalid said.
Former commander killed
Among the victims killed in the first blast was a former senior factional commander, Agha Shah, Kandahar police chief Colonel Mohammad Hakim said.
"I can say now that at least four people, including Agha Shah, have lost their lives in the explosion," Hakim said.
Body parts of victims of the first explosion were strewn on the road outside Shah's house in a crowded area of Kandahar.
The attacks come as Nato plans
to expand its presence
Shah was among local commanders who helped US-led forces overthrow the Taliban government in 2001.
A Taliban commander, who identified himself as Sabir Momin, claimed responsibility for the first incident.
There have been four attacks in less than a week in Kandahar, the Taliban's former stronghold.
The attacks have come as Nato prepares to expand its presence into Afghanistan's restive south and tries to resolve differences among its members about alliance plans to move from peacekeeping to taking command of counter-insurgency operations in Afghanistan.
On Sunday, four British government officials on a visit to help improve customs services in the province were wounded in an attack close to the city centre.
Attacks have been fewer in Afghanistan than those against US-led and Iraqi forces in Iraq, but the Taliban says a number of its members have infiltrated major cities for missions.
The US army says an engine
malfunction caused the crash
The US military also said a US MH-47 special forces helicopter was destroyed after an engine malfunction during an "anti-militant" operation in Kunar on Thursday.
There were no casualties, but the crash was the second of a US military helicopter in Afghanistan in the past two weeks.
Five crew members were killed in the 25 September crash of a CH-47 Chinook in the southern province of Zabul after the aircraft had dropped off troops during an operation against armed fighters.