The United Nations has declared a state of emergency in the Gaza Strip after two days of heavy rains and flooding in the war-battered enclave.

The UN Relief and Works Agency (UNRWA) declared the state of emergency in Gaza City on Thursday, after torrential rain overwhelmed some areas and caused flooding.

"Hundreds of residents in the flooded areas around Sheikh Radwan storm water lagoon have evacuated their homes," the UN agency for Palestinian refugees said, referring to a northern district.

UNRWA said it closed 63 schools and was supplying "emergency fuel to municipalities, water, sanitation and health facilities".

Robert Turner, UNRWA director of operations in Gaza, said the agency was "very concerned about such severe storms this early in the season and on the back of unprecedented damage and destruction" from the war.

"Such devastation exacerbates the already poor humanitarian situation for refugees and non-refugees in Gaza, which is dealing with the aftermath of a recent conflict and an acute fuel and energy crisis".

An estimated 100,000 Palestinians remain homeless in Gaza three months after Israel ended its offensive on the Gaza strip.

Gaza's 1.8 million residents have endured daily electricity blackouts, with major pieces of infrastructure, from roads to sewage treatment plants, still seriously damaged.