Officials said 10 more shells landed in a nearby forest area, damaging trees but causing no loss of lives.
The targeted military post is located several hundred metres inside the Pakistani border.
Last month, 11 Pakistani troops were killed in an air strike by Afghanistan-based US forces.
Growing anger
Officials in Pakistan say the shells were fired by coalition troops operating in the Barmal area on the Afghan side of the border.
An official in Pakistan's South Waziristan province told the BBC Urdu service that the injured had been airlifted to a hospital in Wana, the provincial headquarters.
An army source in Islamabad confirmed the strike and injuries, but offered no further details.
There is increasing anger in Pakistan at US strikes on its territory which have killed more than 50 people this year.
Taleban fighters have a strong presence in Pakistan's border area and tribal districts - local administrators have little power there, although security forces keep a presence on the border.
Afghan and US-led forces accuse Islamabad of not doing enough to deny Taleban militants a hiding place in Pakistan's tribal areas and stop them from infiltrating the border into Afghanistan.
Pakistan denies the accusations, saying it has lost about 1,000 soldiers fighting militants in the tribal border areas.
US strike' hits Pakistan troops
Officials said 10 more shells landed in a nearby forest area, damaging trees but causing no loss of lives.
The targeted military post is located several hundred metres inside the Pakistani border.
Last month, 11 Pakistani troops were killed in an air strike by Afghanistan-based US forces.
Growing anger
Officials in Pakistan say the shells were fired by coalition troops operating in the Barmal area on the Afghan side of the border.
An official in Pakistan's South Waziristan province told the BBC Urdu service that the injured had been airlifted to a hospital in Wana, the provincial headquarters.
An army source in Islamabad confirmed the strike and injuries, but offered no further details.
There is increasing anger in Pakistan at US strikes on its territory which have killed more than 50 people this year.
Taleban fighters have a strong presence in Pakistan's border area and tribal districts - local administrators have little power there, although security forces keep a presence on the border.
Afghan and US-led forces accuse Islamabad of not doing enough to deny Taleban militants a hiding place in Pakistan's tribal areas and stop them from infiltrating the border into Afghanistan.
Pakistan denies the accusations, saying it has lost about 1,000 soldiers fighting militants in the tribal border areas.
