The United States has conducted an airstrike against Taliban fighters in Afghanistan's southern Helmand province, a US forces spokesman said, the first such attack since a troop withdrawal agreement was signed between the two sides last weekend.

The Taliban fighters were "were actively attacking an (Afghan National Security Forces) checkpoint. This was a defensive strike to disrupt the attack," said Colonel Sonny Leggett, a spokesman for the US Forces in Afghanistan in a tweet.

He said Washington was committed to peace but would defend Afghan forces if needed.

"Taliban leadership promised the (international) community they would reduce violence and not increase attacks. We call on the Taliban to stop needless attacks and uphold their commitments," he said.

The airstrike was the first by the US against the Taliban in 11 days, when a reduction in violence agreement had begun between the sides in the lead up to Saturday's pact.

Since the signing, the Taliban had decided on Monday to resume normal operations against Afghan forces, though sources have said they would continue to hold back on attacks on foreign forces.

It has been reported that Taliban militants killed at least 20 Afghan soldiers and policemen in a string of overnight attacks,

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The Taliban has so far declined to confirm or deny responsibility for any of the attacks and did not immediately respond to request for comment on the airstrike.

A Ministry of Defence spokesman said that in the past 24 hours there had been clashes between the Taliban and Afghan security forces in nine provinces, including Helmand.

A spokesman for Helmand's provincial governor said that the Taliban had attacked a security checkpoint in Washer district  - a different district to the one in which the U.S. carried out its airstrike – last night, killing two police officers.

A defence ministry spokesman also confirmed an attack against an Afghan army base in the city of Kunduz and said there were seven casualties.

The weekend agreement envisages a full withdrawal of all US and coalition forces within 14 months, dependent on security guarantees by the Taliban, but faces a number of hurdles as the US tries to shepherd the Taliban and Afghan government towards talks.