KABUL (Reuters) - The United States on Wednesday carried out its first air strike on Taliban fighters in Afghanistan since the two sides signed a troop withdrawal agreement on Saturday.

A U.S. forces spokesman confirmed the incident in southern Helmand province, hours after President Donald Trump spoke by phone with chief Taliban negotiator Mullah Baradar Akhund on Tuesday, the first known conversation between a U.S. leader and a top Taliban official.

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

In a series of posts on Twitter late on Wednesday, U.S. Special Envoy for Afghanistan Zalmay Khalilzad did not refer to the U.S. strike, but said: “Increasing violence is a threat to the peace agreement and must be reduced immediately.”

Khalilzad said the United States was committed to facilitating a prisoner exchange and would “support each side to release significant numbers.”

Other U.S. officials appeared to play down the Taliban attacks.

“There is no attacks on 34 provincial capitals, there is no attacks in Kabul, there’s no high profile attacks, there’s no suicide bombers, there’s no vehicle borne suicide, no attack against the U.S. forces, no attack against coalition,” Chairman of the Joint Chiefs of Staff General Mark Milley said during a hearing.

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“Yes, there were significant numbers of attacks, small attacks. They were all beaten back,” Milley said.

The air strike was the first by the United States 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 will continue to hold back on attacks on foreign forces.

The Taliban has declined to confirm or deny responsibility for any of the attacks.

Taliban spokesman Suhail Shaheen said in a Tweet that “according to the plan (the Taliban) is implementing all parts of the agreement one after another in order to keep the fighting reduced.”

A senior Taliban commander in Helmand who declined to be named said that a drone had targeted their position.

“As far as I know we didn’t suffer any human losses but we are working on it and sent our team to the area,” he told Reuters, adding that the group’s senior leadership in Afghanistan had called an emergency meeting to discuss what he described as a “major violation” of the agreement.

“THINGS COULD SPIRAL”

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Experts said the public agreement was vague on details around ongoing violence in the country, but that the air strike and comments from U.S. officials suggested the United States had a plan to ensure reduced violence against Afghan forces and civilians.

“It is significant. I don’t think it signals the collapse of the whole U.S.-Taliban agreement ... (but) you can easily see how things could spiral,” said Andrew Watkins, a senior analyst covering Afghanistan at International Crisis Group.

A spokesman for Helmand’s provincial governor said the Taliban had attacked a security checkpoint in Washer district - a different district to the one in which the U.S. carried out its air strike - on Tuesday evening, killing two police officers.

An interior ministry spokesman, Nasrat Rahimi, said on Wednesday the Taliban had conducted 30 attacks in 15 provinces in the previous 24 hours, killing four civilians and 11 security and defense force members. Seventeen Taliban members had been killed, he said.

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

In Washington, a Democratic member of the U.S. House of Representatives said secret annexes of the accord contained conditions for the U.S. withdrawal and questioned why they were classified.

“The administration is telling a terrorist group the conditions (such as they are) of our withdrawal from Afghanistan, but not telling the American people,” Tom Malinowski, formerly the State Department’s top official for human rights, wrote on Twitter. “This is wrong. And serves no national security purpose.”