The United States conducted an airstrike on Wednesday against Taliban fighters in Afghanistan's southern Helmand province, a U.S. forces spokesman said, the first such attack since a troop withdrawal agreement was signed between the two sides on Saturday.

The Taliban fighters "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 U.S. 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 United States against the Taliban in 11 days, when an agreement over a reduction in violence had begun between the sides in the lead up to Saturday's pact.

Trump says he had a 'very good conversation' with Taliban leader

U.S. President Donald Trump spoke by telephone with chief Taliban negotiator Mullah Baradar Akhund on Tuesday, the first known conversation between a U.S. leader and a top Taliban official, as a dispute over a prisoner release threatened a U.S.-led effort to bring peace to Afghanistan. 

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The call, announced on Twitter by a Taliban spokesman and then confirmed by Trump, came three days after Baradar and U.S. Special Envoy Zalmay Khalilzad signed an agreement in Qatar for a withdrawal of U.S. forces from Afghanistan. 

That deal, a step toward ending America's longest war, could boost Trump's bid for a second term in the November 3 U.S. election. Trump has made a troop withdrawal from Afghanistan a priority.

The pact calls for a phased withdrawal of U.S.-led foreign forces if the Taliban keeps its commitments and for the March 10 start of talks between the insurgents and an Afghan delegation on a political settlement to end decades of conflict. 

But the peace effort quickly hit an obstacle, after five Afghan policemen died in a Taliban attack on a security checkpoint near a copper mine on Tuesday, a day after the insurgents decided to resume operations against local forces. 

The militants had a reduction of violence arrangement in place before the signing of a troop withdrawal agreement with the United States in Doha on Saturday. 

But on Monday they decided to end that for Afghan forces, while still holding back on fighting American and other foreign troops, according to sources. 

Tuesday's "heavy clash" killing five Afghan officers took place at a checkpoint at the Mes Aynak copper mine in Logar province, said Deedar Lawang, a spokesman for Logar's provincial governor. 

Hasib Stanekzai, head of Logar provincial council, told Reuters the policemen were soft targets during the early morning attack as they had no heavy weapons. 

A senior U.S. official said the attack was being investigated. 

A spokeswoman for Afghanistan's interior ministry said that in 24 hours, the Taliban carried out 33 attacks against Afghan forces in 16 provinces, killing six civilians. She did not say how many Afghan security force members had died. 

"Taliban should give up killing civilians, otherwise (Afghan National Security forces) take action and eliminate them in the defense of our people," said the spokeswoman Marwa Amini.

'A very good conversation' 

Speaking with reporters, Trump gave few details of the discussion but said he had a good relationship with Baradar. 

"We had a very good conversation with the leader of the Taliban today. They're looking to get this ended, and we're looking to get it ended... the relationship is very good that I have with the Mullah. We had a good, long conversation today." 

Trump declined to say if the call was his first with Baradar, a co-founder of the Taliban and its political chief.  

Pakistan released Baradar from prison in October 2018 to lead the Taliban negotiating team. 

The call was the first known conversation between a U.S. president and a leader of the insurgent group overthrown in the 2001 U.S.-led invasion for harboring al Qaeda leader Osama bin Laden while he plotted the September 11, 2001, attacks. 

More than 2,500 U.S. troops have died in Afghanistan and more than 20,660 have been wounded, according to iCasualties, a website that tracks foreign troop casualties in Iraq and Afghanistan. 

Trump in September scotched a first bid to finalize a U.S.-Taliban troop withdrawal deal, citing a U.S. soldier's death in a Taliban attack. Talks resumed in December and the pact was signed on Saturday after a seven-day reduction in violence.

(With input from Reuters)