Five American soldiers were killed by a bomb in Afghanistan Thursday, as the Taliban rejected claims that the fighters who shot down a US helicopter last week had perished in a NATO air strike.

The NATO-led International Security Assistance Force (ISAF) said only that those killed in the blast in southern Afghanistan were the latest victims of the Taliban insurgency's increasing use of crude, home-made bombs.

A Pentagon spokesman in Washington confirmed that all five dead were American troops.

The deaths come a week after the Taliban shot down a US Chinook helicopter, killing 38 people including 30 Americans -- the biggest loss of US life in a single incident since the 2001 invasion.

At least 387 coalition soldiers have now been killed in Afghanistan so far this year, compared to 711 deaths in 2010, according to an AFP tally based on that kept by independent website icasualties.org.

South Afghanistan is the Taliban's heartland and was the focus of a US troop surge from 2010 that commanders say has made significant progress.

But the militia still frequently strike troops on foot patrol or travelling in armoured vehicles with crudely-assembled improvised explosive devices (IEDs).

From April to June, 3,485 IEDs exploded or were found in Afghanistan, according to the Pentagon, up 14 percent from the same period last year.

In addition, officials said five Afghan police were killed in an overnight clash with the Taliban in the southern province of Helmand.

"One of our police posts was attacked in Gereshk last night. Five policemen were killed," said Helmand provincial police chief Abdul Hakim Angaar.

In the capital on Thursday, President Hamid Karzai announced that he would not seek a third term in office.

The Afghan constitution limits a president to two terms and his office issued the statement in response to "rumours" from opponents suggesting he could seek to try and change this rule.

On Wednesday, US General John Allen, commander of the NATO-led international force in Afghanistan, said the Taliban fighters responsible for downing the helicopter last Friday had been killed in an air strike.

But Taliban spokesman Zabiullah Mujahid said that was "not true".

"After seeing the enemy statement, we contacted the mujahed (fighter) who shot down the helicopter and he's not dead. He's busy conducting jihad elsewhere in the country," Mujahid told AFP by telephone from an undisclosed location.

Mujahid admitted that four "ordinary" Taliban fighters had been killed in the US air strike but said they were not the ones who shot down the helicopter.

He added that the fighter responsible had now left Wardak province, where the crash took place in the wild Tangi Valley.

The Taliban are known to exaggerate and distort their public statements as part of a propaganda campaign accompanying their 10-year campaign to evict the mainly US foreign troops who ousted them from power in the 2001 invasion.

Citing intelligence, a senior Afghan official told AFP this week on condition of anonymity that the helicopter was brought down in a Taliban trap designed to lure international forces to the scene.

Allen said the Chinook, which was carrying 25 members of the elite special forces, had been sent in as part of an operation targeting a Taliban leader, who is still at large.

When "less than 10" fighters were seen "escaping", the Chinook was ordered in, Allen said. It was bringing special forces who were to pursue the insurgents.

But the helicopter was shot out of the sky with a rocket-propelled grenade, killing all 38 people on board.

Afterwards, the military said US forces tracked the insurgents responsible, calling in an air strike late Monday with an F-16 fighter.

The air strike killed the "shooter" as well as a Taliban militant, Mullah Mohibullah, as they "were attempting to flee the country in order to avoid capture", ISAF said.

The Pentagon on Thursday released the names of the 30 US troops killed in that attack, saying that 17 of them were elite Navy SEALS and another five were Navy sailors assigned to the SEAL unit.

Those killed in the helicopter crash also included three Air Force special operations forces, the five-man Army crew of the Chinook helicopter, seven Afghan troops and an interpreter.

In a separate incident, two Afghan soldiers were found dead in Logar province Thursday, which borders Wardak in central Afghanistan, after being abducted by insurgents Wednesday, police said.