Story highlights The battle took place near the coalition base in At Tanf, Syria

No coalition forces were killed or wounded the official said

(CNN) US military advisers and their local Syrian partners killed 21 ISIS terrorists and captured 17 more during a battle near a key strategic base Tuesday, two US military officials told CNN.

A military official with the US-led coalition fighting ISIS told CNN that a convoy of about 10 vehicles was passing through the northern portion of the 55km "de-confliction" zone surrounding the coalition base in At Tanf, Syria, a buffer area that was negotiated by Russia and the US-led coalition.

US-backed local Maghawir Al-Thawra fighters accompanied by US advisers moved to intercept the convoy and identify it. When they discovered that the convoy included ISIS fighters, an intense firefight broke out with about 15 ISIS fighters killed in the initial battle. No coalition forces were killed or wounded, the official said.

The US-led coalition, Operation Inherent Resolve, issued a statement later Thursday saying over 20 ISIS fighters were killed in the battle.

Two US military officials told CNN the number was 21 killed and 17 captured.

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