SUSPICIOUS grassfires in Melbourne's south-east are believed to have been deliberately lit.

Two fires at Berwick and Clyde North yesterday have sparked a police investigation with authorities urging witnesses to come forward.

The first fire at Berwick occurred about 7.45pm at Sweeneys reserve, with a second flare about 200m away discovered along drain line amongst trees.

Police and CFA were also called to Clyde North about 8.50pm.

Police said emergency services personnel found up to nine grass fires at different locations along the road side, spread 500m apart.