The investigation was opened after lab tests discovered the traces in the food at a base run by the Nato-led International Security Assistance Force (Isaf) in Nangarhar province, an Isaf spokesman told AFP.
"There were no injuries, no fatality. The investigation is ongoing," Master Seargent Nicholas Conner said, adding that Nato staff, Afghans and nationals from a third country worked at the dining facilities.
The Taliban, which is waging an insurgency to oust the 130,000-strong US-led Isaf force from Afghanistan, claimed that an "Afghan cook" had poisoned the food.
The incident follows days of anti-US protests over the burning of Holy Quran at a US military base and the killing of two senior US advisors inside the nation's interior ministry compound, reportedly by an Afghan employee.
Two other US soldiers were killed, also in Nangarhar, by an Afghan trooper who turned his weapon on them as demonstrators approached their base last Thursday.
And on Monday, a Taliban suicide car bomber killed nine people in an attack on the airport in the provincial capital Jalalabad, where another Nato base is located.
No Isaf staff were injured in the blast, which the Taliban said was in retaliation for the Holy Quran burning at the US-run Bagram airbase north of Kabul.
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