KANDAHAR, Afghanistan (AP) — An Afghan police officer and cook poisoned their colleagues at a checkpoint in an assault coordinated with insurgent fighters that left six dead in the country's south, officials said Saturday.

It was the latest in a string of attacks from inside the Afghan army and police that are threatening to undermine both the partnership with international troops — which have been the target of many attacks — and the morale of Afghan forces, who have suffered equally heavy casualties from such strikes.

The police officer and the cook worked with outside insurgents in the assault, which hit police manning a checkpoint in the Gereskh district of Helmand province, the governor's office said in a statement.

They poisoned two of the officers and then the militants attacked from outside, killing the remaining four officers, provincial spokesman Ahmad Zirak said. He did not say how the officers were poisoned. The police officer was captured as he fled, but the cook escaped and remains at large, Zirak added.

The insurgent gunmen escaped by motorcycle with weapons and ammunition, the governor's statement said.

A recent upsurge in the number of insider attacks on coalition troops by Afghan soldiers or police — or insurgents disguised in their uniforms — has further undermined public support for the war in the West. So far this year, at least 52 foreign troops — about half of them Americans — have been killed in insider attacks.

The Afghan government has not provided statistics on the number of its forces killed in insider attacks. However, U.S. military statistics obtained by The Associated Press show at least 53 members of the Afghan security forces had been killed by the end of August.

Meanwhile, a Taliban attack elsewhere in Helmand killed two district community council members, while Taliban-fired rocket-propelled grenades destroyed a warehouse full of food destined for the main U.S. base in Afghanistan.

Insurgents ambushed the council members while they were driving to a tribal meeting in the volatile Sangin district, the governor's office said in its statement, adding that the attackers escaped and police are pursuing them.

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The attack against the council members is a reminder of the other worrying trend in insurgent tactics this year — a shift toward more targeted killings of those affiliated with the government. The United Nations has recorded a sharp increase in such killings in the first six months of 2012 as compared with the same period of 2011.

In the warehouse attack, insurgents fired rocket-propelled grenades at a compound used by military contractor Supreme Group to store food and other supplies destined for Bagram Air Field, the main U.S. base in the country. A warehouse inside the compound caught fire in the assault and burned through the night.

"The local fire brigade attended the scene and brought the fire under control, but the warehouse itself and all contents were destroyed," Victoria Frost, a spokeswoman for Supreme Group in Dubai, wrote in an email. She said no one was injured and staff at the site did not have to evacuate.

The fire could still be seen burning Saturday morning, said Mohammad Asif, the deputy administrator for Bagram district, where the compound is located. He said the Supreme compound encompasses about five hectares (12 acres).

Frost said the fire was contained much earlier.

"As with any major fire, there are some areas still smoldering but there is no current danger to any of the staff or the other buildings within the compound," she said.

Taliban spokesman Zabiullah Mujahid said in an email that the fire at the Supreme Group compound destroyed a "large stock of food meant for U.S. troops."

Frost did not say how much material was destroyed though she did say it was "primarily food supplies," adding that the company was working to make up the loss with inventory from other warehouses.

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Vogt reported from Kabul. Associated Press writer Rahim Faiez contributed to this report in Kabul.