Taliban insurgents overran an Afghan National Army base near Asadabad Sunday morning, killing 21 soldiers in their bunks in what appeared to be the worst single blow to government forces since 2010, according to both government and insurgent officials.

President Hamid Karzai ordered an investigation and canceled a planned state visit to Sri Lanka in response to the attack, in the Ghaziabad district of Kunar province, near the eastern border with Pakistan.

The attack highlighted the vulnerability of Afghan military units, which are generally no longer accompanied by U.S. or other NATO advisers and do not have the close air support they often enjoyed. And it raised questions about the Afghans’ ability to hold out against the insurgents on their own as the NATO mission winds down and international forces prepare to leave Afghanistan at the end of 2014.

At the same time, there were new signals that efforts to start peace talks with the insurgents were foundering. In an unusual statement released Sunday, the Taliban acknowledged that it had suspended talks with the Americans aimed at a prisoner exchange: the release of five Taliban prisoners held at the Guantanamo Bay prison camp in exchange for the lone American prisoner of war held by the Taliban, Sgt. Bowe Bergdahl.

A Taliban spokesman, Zabiullah Mujahid, said that the talks had taken place with the mediation of Qatar, but that the insurgents had broken them off because of the “complicated political situation” in Afghanistan. He did not elaborate.

The governor of Kunar province, Shuja al-Mulk Jalala, said it appeared that infiltrators had let the Taliban insurgents into the base around 4 a.m., and that most of those who died had been killed in their sleep. Jalala put the death toll at 20, with eight other soldiers reported to have been taken prisoner by the insurgents.

Based on the latest statistics available, through last June, army fatalities had more than doubled compared with the previous year, coinciding with a steady decrease in American and NATO forces in country.

“This incident is so painful and upsetting, and why did it happen?” said Mirdad Khan Nejrabi, head of the Afghan Parliament’s internal security committee.

He criticized Karzai for appearing to show more concern for Taliban prisoners than for his own troops in the field.