Health authorities in the Hamas-run Gaza strip have declared a polio epidemic in the war-ravaged enclave, blaming it on Israel's military offensive.

The Gaza health ministry owed the situation to the scarcity of clean water and food, the overcrowding of residents and the destruction of the sewage system as well as the accumulation of "thousands of tons of garbage."

It said it detected Variant Poliovirus type 2 (CVPV2) in the sewage water in Khan Younis and the central province in Gaza.

Polio had previously been eradicated in the Palestinian territories, the health authorities added.

Last week, the UN's World Health Organization said it was "extremely worried" about a polio outbreak, after traces of the disease were found in sewage samples collected from two sites in the strip.