The bullets recovered today were of 7.62 mm calibre, usually used in assault rifles such as AK-47s.
A special police team formed to probe the recovery of landmines on January 5 stumbled upon the unusual cache of ammunition, which included pulse generator cables used to transmit electrical signals, magazine clips and empty bullet cases.
"We recovered the cartridges while we were probing the earlier incident of landmines. Some eyewitnesses led us to this particular area and further probe led us to these cartridges under water," Palakkad district police chief Prateesh Kumar told NDTV.
"It's a major concern because these are used only by security forces and should not be found in civilian areas. Even the landmines recovered earlier, we have confirmed, were manufactured at the ordnance factory in Maharashtra," he said.
Police said they are probing all angles as to how the cache of ammunition came to a civilian area.
Five anti-personnel mines, two metal parts and six gunny bags were found scattered under a bridge on the banks of Bharathapuzha river on January 5.