Six Afghan private security guards were beheaded during a bank robbery in northern Afghanistan, police said Tuesday.

Also Tuesday, insurgents launched a ground attack on NATO’s largest base in the south, but did not breach its defences, officials said.

It appeared that the security guards were poisoned before they were beheaded, said Sherjan Durani, a police spokesman for Balkh province. He said someone apparently mixed a poison into the guards’ food on Monday night at a branch of Kabul Bank in the northern city of Mazar—e—Sharif.

Mr. Durani says an unknown number of robbers beheaded the guards and took about $269,000 in U.S. and Afghan currency. The case is being investigated.

The one—hour assault on Kandahar Air Field started just before midday and lasted about an hour before the attackers fled, said Maj. Fred De Mos, a spokesman for NATO forces.

No one immediately claimed responsibility for the attack, but it fits the pattern of recent Taliban attacks against high—profile government and military targets.

Taliban insurgents previously tried to storm the Kandahar base on May 22, just a few days after the insurgents sent suicide bombers to assault the main U.S. base in the country, Bagram Air Field near the capital.

Mr. De Mos said he did not yet have any information on casualties, nor how many assailants there were. He said troops were still chasing down insurgents in the area around the base but the installation was no longer under threat.

Kandahar provincial government spokesman Zalmai Ayubi said there were five attackers, two of whom were killed in the attack. He said they fired two rockets into the base, but did not have further details.

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