The KIA is the last ethnic insurgent group still fighting Myanmar's government after a 17-year ceasefire broke down in 2011 and a peace process to end conflict with ethnic minorities in Myanmar has stalled.
US President Barack Obama called on Myanmar's leaders to push forward with the peace process on a visit to the country last week.
The number of dead in the shelling was the largest made public in the conflict between the KIA and army for many months.
In April, General Gun Maw, deputy commander in chief of the KIA and a member of the insurgents' main political committee, said that the KIA had suffered over 1,000 casualties since hostilities resumed, including 280 killed.
The KIA took up arms in 1961 and is the second largest among the 20 ethnic armed groups in Myanmar.