Israel kept a commercial crossing with Hamas-ruled Gaza shut for a seventh day on Tuesday although a truce had stopped cross-border fighting, and a UN official said he was "extremely worried" essential supplies may run out.

The United Nations Relief and Works Agency (UNRWA) which provides aid to more than two thirds of Gaza's population of 1.5 million, said 172 truckloads of oil, sugar and flour were waiting to cross into the impoverished coastal territory.

Israel said it shut the crossings a week ago during a violent flare-up when Hamas militants fired an anti-tank rocket at a school bus, critically wounding an Israeli teenager, and Israel retaliated with air raids, killing 19 Palestinians.

The violence has subsided since Egyptian and UN mediators achieved an informal truce on Sunday.

Israel has not yet reopened the terminal because of concern about security, an Israeli official said, adding that individual humanitarian cases were allowed into Israel at a separate crossing.

Christopher Gunness, an UNRWA spokesman, said he was "extremely worried" the commercial crossing at Kerem Shalom might not reopen before the Jewish Passover holiday begins on Monday evening, a time when Israel often shuts its crossings with Palestinian territories, citing security concerns.

The relief agency generally aims to send about 20 truckloads of sugar, flour, oil and school lunches into Gaza daily, and has only enough of these supplies stored in the territory to last until the end of the month, Gunness said.

The Gaza-based Palestinian Center for Human Rights (PCHR) said 150 types of basic medicines were lacking in the territory and that a cooking gas shortage also loomed.

Israel denied there were shortages of any basic items in Gaza.

Gaza shares a border crossing with Egypt at its southernmost point, Rafah, but commercial goods are brought in only via a monitored Israeli terminal, though smugglers bring in goods through tunnels dug beneath the desert frontier with Egypt.

Four Palestinians from Gaza were killed on Tuesday trying to smuggle in cooking gas through such a tunnel that collapsed. It took rescuers several hours to dig out their bodies, the civil emergency services in Gaza said.