The soldiers were on a routine patrol when a bomb planted by suspected pro-Taliban militants exploded on a roadside, a local military spokesman told AFP.
"The death toll is two now, as an injured soldier died at the hospital," the official said.
He earlier gave initial toll of one dead and one injured.
Separately four people including a tribal elder were shot dead by unknown attackers in different areas of Swat valley early Sunday, police said.
"All of them were shot dead at their homes by unknown attackers," senior police official Fazal Subhan told AFP.
Residents, however, suspected pro-Taliban militants behind the shootings.
The government signed a peace deal with the militants of Swat in May and agreed to gradually pull out troops and introduce an Islamic justice system. In exchange, the rebels said they would halt attacks and surrender their arms.
The Pakistan army launched a major offensive in October to clear mountainous Swat of militants loyal to Maulana Fazlullah, a radical pro-Taliban cleric who led an uprising to enforce Sharia law in the valley.
Pakistani forces on Sunday pressed ahead with an anti-militant offensive in Khyber tribal district, near the Afghan border.
The operation is the first by the new government since it began negotiating with Taliban militants after elections in February - although officials said the targeted rebel warlord, Mangal Bagh, had no direct links to the Taliban.