A small Israeli force has entered Gaza for the first time since the capture of one of its soldiers last week even as Palestinian groups set Israel a deadline for the release of prisoners.
Tanks and armored vehicles crossed the northern Gaza border, along with a detachment of Israeli troops, to search for explosive devices and tunnels, militiary sources said on Monday.
Witnesses said around 25 Israeli tanks crossed into Gaza. However, Israeli military sources said the incursion was not a large-scale ground operation and would be limited in duration.
The Palestinian groups that abducted the Israeli soldier gave Israel time until 0300 GMT on Tuesday to meet their demands, threatening unspecified consequences if it refuses.
"Military Communique 3" issued on Monday by Hamas's armed wing and two other factions said: "If the enemy does not agree to our humanitarian demands ... we will regard this case as closed."
In previous statements, the groups demanded that Israel, as a first stage, release Palestinian women and youths in its prisons in exchange for information about Corporal Gilad Shalit, seized in a cross-border raid on June 25.
The groups -- Hamas's Izz el-Deen al-Qassam Brigades, the Popular Resistance Committees and the previously unknown Islamic Army -- have also called on Israel to free 1,000 Palestinian prisoners.
Operation intensified
Also on Monday, an Israeli helicopter gunship fired a missile into a Hamas office in Gaza City, damaging a building the Israeli army said was being used by Palestinian fighters.
However, President Mahmoud Abbas' Fatah movement said the office was a charity and near to a Hamas office. There were no reported casualties.
Witnesses also reported two missiles landing in open areas in northern Gaza, near the town of Bait Lahiya.
Monday's strike came hours after Ehud Olmert, the Israeli prime minister, ordered the military to intensify its operation to free a captured Israeli soldier.
Olmert told Israel's weekly cabinet meeting that his government would not bow to "blackmail" in order to rescue Shalit.
"My government has instructed the IDF (Israeli military) and the security establishment to do everything in order to bring Gilad back home... and when I say everything, I mean everything," he said.
About 5,000 Israeli troops remain poised on the Gaza border in the largest Israeli military operation since it pulled out of the tiny coastal territory last year, ending a 38-year occupation.
Photo Caption
An Israeli tank