Colonel Mohammed Israr, an Afghan battalion commander in Khost, said that the group of about 50 Taliban fighters had crossed from Pakistan and retreated in that direction under heavy coalition fire.
However, Lieutenant Nathan Perry, the coalition spokesman, said the clash took place about eight kilometres from the border and "did not involve Pakistan".
Clashes between Taliban fighters and security forces are intensifying in the southern half of Afghanistan. Since the beginning of the year, more than 2,100 people have died in the violence, according to an Associated Press tally.
Taliban fighters have suffered the heaviest losses, but casualties among foreign troops are also rising.