Israel stepped up its military offensives in northern Gaza Strip on Tuesday, as the operations aimed at bringing back the kidnapped Israeli soldier Gilad Shalit entered the sixth day.

Infantry troops of the Israel Defense Forces (IDF), backed by armored personnel carriers, tanks and bulldozers, moved into the Erez industrial zone in northern Gaza Strip Tuesday afternoon to search for tunnels and explosive devices used by the Palestinian militants to attack Israel, Israeli army sources said.

"The tunnel threat still exists, not only for IDF, but for Israeli communities near the strip as well," the sources said.

Earlier on Monday, the IDF also sent a small number of forces into northern Gaza to conduct "a pinpoint operation" in the area near the border fence to reveal explosives and tunnels dug into the ground by Palestinian militants, said an IDF statement.

IDF's Tuesday invasion into northern Gaza came hours after the expiration of a 24-hour ultimatum issued by the kidnappers of Shalit, who was abducted in a Palestinian tunnel-borne attack on an Israeli army post on June 25.

On Monday, the three Palestinian militant groups holding Shalit set 6:00 a.m. (0300 GMT) Tuesday for Israel to free hundreds of prisoners in exchange for the Israeli soldier, which has been rejected by Israel.

The Israeli Prime Minister's office said on Monday that Israel would not give in to Palestinian militant's blackmail and refused to hold any negotiation on the release of prisoners.

In addition, Israel also held Palestinian National Authority ( PNA) fully responsible for the safety of Shalit and his release.

Following the expiration of the ultimatum, Palestinian militant factions behind Shalit's kidnapping has announced a close of negotiations with the Egyptian mediators for the soldier's release.

Meanwhile, Israel reiterated Tuesday that its refusal to negotiate with the militants holding Shalit remained unchanged.

Israeli Prime Minister Ehud Olmert and Defense Minister Amir Peretz decided on Tuesday to step up military operations in northern Gaza Strip, but did not give the actual timing for the action.

Olmert visited the southern Israeli town of Sderot Tuesday morning and met with municipal officials there.

During the meeting with Sderot officials, the prime minister vowed to continue targeting the infrastructure in Gaza unless the Palestinian militants stop firing rockets at southern Israel.

Palestinian militants fired three rockets into southern Israeli city of Negev from northern Gaza Strip in the morning, one of which landed near kibbutz Yad Mordechai and two others in open fields near kibbutz Nahal Oz. There was no report about injury and damage.

Besides ground offensives into northern Gaza, air raids and artillery shelling by IDF continued on Tuesday.

An IDF statement released on Tuesday said that two airstrikes were carried out in northern Gaza in the day, one of them was targeting Palestinian militants who were planting explosive devices while the other was aiming at a structure used by the Islamic Resistance Movement (Hamas) to plan attacks.

Source: Xinhua