US military vehicles block the road following an attack by Taliban on a foreign compound in Kabul, Afghanistan on Aug. 8, 2015. (Xinhua/Ahmad Massoud)

The US launched an airstrike against Taliban fighters to defend Afghan forces Wednesday, an American military spokesman said, as violence spirals out of control after a string of deadly attacks by insurgents, throwing the country's fragile peace process into grave doubt.

News of the airstrike in southern Helmand province - the first in 11 days - came hours after US President Donald Trump told reporters that he had had a "very good" chat with the Taliban leader, who on Saturday signed a historic deal with Washington to withdraw foreign forces from Afghanistan.

Since the signing in Doha however, the militants have ramped up violence against Afghan forces, ending a partial weeklong truce that provided a rare reprieve to war-weary residents.

US Forces-Afghanistan spokesman Sonny Leggett tweeted that the airstrike took place against Taliban fighters who were "actively attacking" an Afghan forces checkpoint in Helmand province.

"This was a defensive strike to disrupt the attack," he tweeted.

"We call on the Taliban to stop needless attacks and uphold their commitments. As we have demonstrated, we will defend our partners when required."

He said insurgents had carried out 43 attacks on checkpoints in Helmand on Tuesday alone.

The insurgents killed at least 20 Afghan soldiers and policemen in a series of overnight attacks, government officials told AFP Wednesday.

"Taliban fighters attacked at least three army outposts in Imam Sahib district of Kunduz last night, killing at least 10 soldiers and four police," said Safiullah Amiri, a member of the provincial council.

A defense ministry official speaking to AFP on condition of anonymity confirmed the army death toll, while the provincial police spokesman Hejratullah Akbari confirmed the police fatalities.

The insurgents also attacked police in central Uruzgan province Tuesday night, with the governor's spokesman Zergai Ebadi telling AFP: "Unfortunately, six police were killed and seven wounded."

AFP