(CNN) The crowd of soldiers barely had time to finish their prayers before the Taliban came to kill them.

Atiqullah was one of soldiers praying at the northern Afghan army base. As he left, he saw the carnage unfold.

"I saw a ranger model military vehicle with three or four attackers. Two of them had suicide vests -- they blew themselves up," said Atiqullah, a member of the 209th Shaheen Corps.

Another similar vehicle "went to mosque side -- they had M16 weapon(s) and started firing at anyone who came in front of them."

By the end of the bloodshed Friday, "I saw more than 80 to 100 people killed in front of me," the 25-year-old soldier said.

A wounded Afghan National Army soldier lies a hospital in Mazar-i-Sharif on Saturday.

Officials have not released an exact death toll, but sources close to the situation told CNN as many as 140 people may have been killed in the attack.

Now, one big question remains: How did the insurgents get so close to the soldiers inside the base?

"If I forget my ID card, they don't allow me to go inside," Atiqullah said. "How can a suicide attacker go inside?"

Officials said the Taliban fighters were dressed in military uniforms. But it's not clear whether that alone helped them get in.

An act of revenge

The assailants terrorized Camp Shaheen near Mazar-i-Sharif , one of the country's most populated and developed cities, the Afghan military said.

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Atiqullah and another soldier, Mohammad Hassan, said they suspect the terrorists might have had contact with people inside the base.

"There are eight gates at the corps," said Hassan, a 30-year-old commando soldier wounded in the attack. "I request the government to punish those who helped those terrorists."

The Taliban claimed responsibility for the deaths. In an email to CNN, spokesman Zabiullah Mujahid said it was retaliation for the Afghan forces killing two Taliban officials in the northern provinces of Kunduz and Baghlan.

The slaughter at the base's mosque and in a dining facility lasted arounf six hours. By the end, at least five attackers were killed and one was arrested, Afghan army spokesman Abdul Qahar Araam said.

Afghan President Ashraf Ghani declared Sunday to be a national day of mourning in honor of the Afghan forces killed.

Pres @ashrafghani visited injured soldiers & paid tribute to the martyrs of the ANA 209th corps pic.twitter.com/ji5Ng4cRib — ارگ (@ARG_AFG) April 22, 2017

The commander of Resolute Support -- the NATO-led mission to train and assist Afghan forces -- said the attack "shows the barbaric nature of the Taliban."

"They killed soldiers at prayer in a mosque and others in a dining facility," Gen. John Nicholson said. He said Afghan soldier and security forces "have my personal assurance that we will continue to stand with them."

Thousands of soldiers killed

From January 1 through November 12 last year, 6,785 Afghan national security forces were killed, according to the latest quarterly report of the special inspector general for Afghanistan reconstruction.

"The numbers of the Afghan security forces are decreasing, while both casualties and the number of districts under insurgent control or influence are increasing," according to the January 30 report to Congress.

Even those in peaceful prayer aren't immune from attack.

"People went to offer Friday prayer, but enemies of the country killed most of them," Atiqullah said.

Trying to break a 'stalemate'

The attack on the Army base is a huge blow to the Afghan government and its coalition allies grappling with the relentless Taliban insurgency and the rise of ISIS in Afghanistan.

US troops have been fighting a resilient Taliban there for almost 16 years . American forces enjoyed a brief victory last week, when the US military said Taliban leader Quari Tayib was killed in an airstrike in Kunduz province.

In February, Gen. John Nicholson , commander of US forces in Afghanistan, told the Senate Armed Services Committee that leadership assesses "the current security situation in Afghanistan as a stalemate."

There are 8,400 US troops in Afghanistan and 6,000 troops from NATO and allied counties. Nicholson said the coalition faces "a shortfall of a few thousand troops" to break the stalemate.

The Friday attack by the Taliban came more than a week after the US military dropped America's most powerful non-nuclear bomb on ISIS targets in Afghanistan, killing 94 ISIS fighters.

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ISIS claimed responsibility for an attack last month on Kabul's heavily fortified diplomatic quarter.

National security adviser Lt. Gen. H.R. McMaster recently visited Afghanistan as Washington considers a full "strategy review." of policy toward that country.

"In recent years, at a period of our maximum effort, we didn't have as reliable a partner in the Afghan government as we would've liked," McMaster said.

"Now, we have a much more reliable Afghan partner and we have reduced considerably the degree and scope of our effort."