KABUL (Reuters) – Afghanistan's Taliban on Tuesday accused the United States of hampering peace negotiations in response to the top American diplomat's comments that a reduction in violence was needed before a deal to end years of war could be struck.

U.S. Secretary of State Mike Pompeo on Monday said during a visit to Uzbekistan that “demonstrable evidence” of a reduction in Taliban violence was necessary for a peace agreement with the Islamist group.

Taliban spokesman Zabihullah Mujahid said issues from the American end, including a tweet by U.S. President Donald Trump halting the signing of an agreement last year, were the reason for challenges in the peace process.

“Mr. Pompeo should not shift the blame. Our stance is principled and united, and our policy is not shaky like the opposite side,” said Mujahid in a statement on messaging service Whatsapp.

Trump abruptly called off the talks to end the 18-year war in a Tweet in September after a U.S. soldier was killed in an attack by the militant group.

Talks have since resumed but have suffered setbacks due to multiple attacks, including a December suicide bombing of a U.S. base outside Kabul that killed two civilians.

Hostilities have surged in recent weeks, suggesting a deadlock in the peace talks between U.S. and Taliban negotiators in Qatar's capital, Doha.

At least 29 members of the Afghan security forces were killed in Taliban attacks that followed air and ground assaults by government forces on the Islamist group in January.

Afghan forces and the Taliban also clashed last week when security personnel tried to access the site of a crashed U.S. military plane in central Afghanistan. U.S. forces were later able to get to the site and recover the remains of two personnel and what is believed to be the flight data recorder.

Trump was reported last year to be planning to withdraw around half of the 14,000 U.S. forces in Afghanistan, adding pressure to secure a peace agreement with the Taliban to prevent the country from collapsing.

The Taliban have made increasing gains as moves towards a possible peace deal have continued, with expanding control over large parts of the war-torn country.