The warnings came three weeks after Israel and Hamas signed a ceasefire to end 50 days of conflict, and as the Israeli army said a mortar fired from Gaza hit southern Israel -- the first since the truce.
"Without immediate action by the Palestinian Authority, donors and the government of Israel to revitalise the economy and improve the business climate, a return to violence as we have seen in recent years will remain a clear and present danger," the World Bank quoted Palestinian territories director Steen Lau Jorgensen as saying in a statement.
Israel's blockade has restricted and sometimes entirely prevented certain building materials from entering Gaza, with the Jewish state saying they could be used by militants to make weapons or build attack tunnels.
The IMF said the war's economic impact was difficult to evaluate, but that the level of destruction and limited resources in Gaza required a "fundamental change of the political status quo," and significant provision of international aid.
It said if Israel's blockade on Gaza was not lifted, partially or entirely, including allowing in building material, reconstruction efforts could fail.