An Israeli tank drives a long the border with the Gaza Strip, seen in the background, 17 Nov. 2008

Palestinian witnesses say Israeli tanks have moved into the Gaza Strip, further threatening a fragile cease-fire Israel signed with Hamas militants in June.

The witnesses say the tanks leveled lands Tuesday along the southern Gaza border east of the city of Rafah.

No casualties have been reported.

Meanwhile, Israel says it will keep all border crossings with Gaza closed today because of Palestinian rocket fire into southern Israel.

Militants fired at least six rockets Monday at towns in southern Israel.  No casualties were reported.

Israel shut its border crossings with Gaza after fighting erupted 12 days ago between Israeli troops and Gaza militants.  On Monday, Israeli Defense Minister Ehud Barak said he will keep the crossings closed because of the attacks.

The United Nations and the European Union are urging Israel to ease the blockade, which has led to shortages of food and fuel in the aid-dependent Palestinian territory.

Israel allowed a humanitarian aid convoy of 33 trucks into Gaza on Monday.

Some information for this report was provided by AP.