Israel resumed shelling the Gaza Strip, less than an hour into a temporary ceasefire Sunday, saying the fire was in response to continued attack by Hamas.
Lt.Col. Peter Lerner, spokesperson for the Israeli army, said in a statement that "once again Hamas breaches ceasefire... the Israeli Defence Force is responding accordingly".
Hamas blamed Israel for initiating the fire and bombing medical teams, according to a Twitter account associated with the Islamist organisation.
Earlier Sunday, Israel agreed on a request by the International Red Cross Committee to a two-hour humanitarian truce, starting at 1:30 p.m., in order to allow medical teams in Gaza to evacuate the dead and wounded to hospitals.
At least 40 Palestinians were killed and 400 others wounded early Sunday by Israeli artillery fire in the Gaza city, making it the heaviest casualties since Israel launched its ground operation late Thursday night.
Palestinian health ministry officials said at least 390 Palestinians, many of them civilians, have been killed in the 13-day conflict and about 3,000 have been wounded. On Israel's side, two civilians were killed in cross-border firing and five soldiers died as the conflict intensified.