"Seven Frontier Corps soldiers embraced shahadat (martyrdom)," a military statement said. It was not immediately known whether the fire came from militants or from Afghan security forces.
It said the heavy fire from across the border targeted a checkpoint northeast of Angoor Adda in South Waziristan tribal region, AFP reported.
Pakistan's seven tribal districts near the Afghan border are rife with homegrown insurgents and are strongholds of Taliban and Al-Qaeda operatives.
Local militants have fled ongoing military offensives since 2008, taking refuge across the porous border from where they have launched attacks on Pakistani military checkpoints and civilians.
The latest attack came two months after a cross-border rocket attack by militants on a Pakistani checkpoint in Khyber tribal region killed at least four soldiers and wounded four others.
In 2011, 24 Pakistani soldiers were killed in a NATO air strike on the Afghan border that sent tensions soaring, with Islamabad cutting off critical NATO supply routes into Afghanistan and Pakistani officials saying the US had deliberately targeted their troops at border posts.
In June last year the army launched a long-awaited offensive against militants in the tribal areas of North Waziristan, where they had previously been operating with impunity.
Islamabad and Kabul regularly accuse each other of giving sanctuary to militants. Afghanistan in particular accuses Pakistan of supporting Afghan Taliban insurgents.
Pakistan has been battling a homegrown insurgency for over a decade following the late 2001 US-led invasion of Afghanistan.