Black smoke is seen during an Israeli bombardment over the town of Beit Lahiya, northern Gaza Strip July 8, 2006. UN chief Kofi Annan called for an immediate stop to what he called Israel's 'disproportionate use of force' in Gaza but also pressed for the release of an Israeli soldier snatched by Palestinian militants.[Reuters]

Israel launched rapid-fire air strikes against Palestinian militants across the Gaza Strip on Sunday after rebuffing a proposed ceasefire by Prime Minister Ismail Haniyeh of Hamas.

Israel is threatening to expand its offensive unless militants release Corporal Gilad Shalit, an Israeli soldier abducted in a cross-border raid on June 25, and halt rocket fire on Israeli cities.

At least three militants were injured in an early morning air strike near the Karni commercial crossing, Palestinian medics and police said.

Israel also bombed a key bridge in northern Gaza, despite an appeal from the United Nations to stop targeting the strip's fragile infrastructure.

The army said the goal was to prevent militants from transporting rockets to launch sites.

In southern Gaza, Israel bombed a militant training camp. No one was hurt. "It was being used to direct attacks," an army spokesman said.

Israel killed seven Palestinians, including a 6-year-old girl and a policeman, on Saturday, Palestinian witnesses said.

About 50 Palestinians -- and one Israeli soldier -- have been killed so far in the offensive, launched on June 28 to pile pressure on the Hamas-led Palestinian government, already on the brink of financial collapse from a Western aid embargo.

Israeli tanks withdrew from much of the northern Gaza Strip on Saturday but soldiers remain in the southern part of the territory and have deployed close to the outskirts of Gaza City.

"We will soon begin operations in other places," Israeli Military Southern Command Chief Yoav Galant told Israeli media. "I see no reason to stop the offensive as long as they are holding our soldier."

Galant said the Israeli offensive, criticized by the European Union and United Nations, had no time limit and that troops were prepared to continue it for "a month, two months and more if needed."

UN Secretary-General Kofi Annan called both sides to exercise restraint and respect their obligations under international law.

"As I have repeatedly stated, I am extremely concerned about the dangerous situation in the occupied Palestinian Territory. I am appealing for urgent action to alleviate the desperate humanitarian situation of the civilian population," Annan said.

"The Israeli air strikes on Gaza's only power plant have had a far-reaching impact on Gaza's hospitals, flour mills, water and sanitation systems," he said.

Haniyeh said Israel's withdrawal from the Gaza Strip and a halt to military actions could "make it easy for us to end the crisis" over Shalit.

Israel said it would only pull its troops out if militants halted rocket attacks and released the soldier.

The Islamist Hamas and other militant groups have demanded Israel make the first move by releasing hundreds of Palestinian prisoners in exchange for the soldier. The Jewish state has rejected this.

"Our intentions are to bring security for Israel citizens and to release Gilad," Galant said. "If someone expects us to play into the hands of the Palestinians, they are mistaken."