Palestinian militants in the Gaza Strip bombarded southern Israel with mortars and rockets on Wednesday, burdening diplomatic efforts to revive a truce that expired over the weekend.

A civilian man who works for a conflict resolution center was badly wounded in an explosion at a house in Gaza City. Two other civilians were lightly hurt when a rocket failed to clear the border and landed on a house in the northern town of Beit Lahiya, Gaza health officials said. Separately, two militants were killed when an explosive they were preparing went off prematurely.

Hamas, the Islamic militant group that rules Gaza, said the bombardment came in retaliation for the deaths of three of its fighters in a clash with Israeli troops late Tuesday. Israel said the militants were planting explosives in northern Gaza along the border fence.

The Israeli military said nine mortars and at least 13 rockets were fired at southern Israel early Wednesday. No injuries were reported. But near Gaza City, an explosion tore through a two-story apartment building, badly wounding Iyad Dremly, an attorney who works for the Palestinian Center for Conflict Resolution. Militants were firing rockets and mortars from the area, but the military said it did not carry out any attacks on Gaza, suggesting the blast was caused by misfired explosives.

Gazans living in border areas criticized militants for operating within residential areas, but would not agree to be quoted for fear of reprisal.

Before the violence resumed, Israel had agreed to crack open cargo crossings with Gaza on Wednesday to allow in a limited amount of food, medicines and fuel, including supplies from Egypt. But military spokesman Peter Lerner said the passages would remain closed in light of the militant barrages.

Israel has maintained a strict blockade of Gaza since the cease-fire began unraveling six weeks ago, allowing in only small quantities of essential goods. Egypt has similarly sealed its border crossing with the territory.

The sanctions have deepened the destitution in Gaza, home to 1.4 million Palestinians who are confined to the tiny coastal strip. Gazans have worked around the choking off of supplies by bringing in goods through tunnels dug under the Gaza-Egypt border.

So far the number of rockets and retaliatory Israeli airstrikes has not approached the pre-truce level, feeding hopes that the cease-fire can be resumed. Both sides have expressed willingness to consider reviving it.

Egypt, which mediated the expired truce, is leading the diplomatic efforts to renew it. On Thursday, Israeli Foreign Minister Tzipi Livni is to meet with Egyptian President Hosni Mubarak in Cairo.

Alongside talk of restoring the truce, Israel is preparing for an escalation of violence.

Israeli leaders have approved a large-scale military operation to stop the rocket fire, but are reluctant to press ahead with a campaign sure to exact heavy casualties on both sides. Past incursions have not halted the barrages, and defense and political officials fear anything short of a reoccupation of Gaza would fail to achieve the desired results.

Israel left Gaza in 2005 after a 38-year occupation.