Qassam rockets fired by Gaza militants hit near Ashkelon and Netivot on Thursday, widening the scope of cross-border violence that has threatened to unravel a shaky Israel-Hamas truce.

The rocket that struck Ashkelon hit an open area near the southern town. No one was wounded and no property damage was caused in the attacks.

Earlier Thursday, The Israel Defense Forces announced it would delay the replacement of its top Gaza commander due to recently the increased tensions on the border.

Brigadier-General Moshe (Chico) Tamir was scheduled to be replaced by Brigadier-General Eyal Eisenberg on Friday, but a military official announced that the prior's term has been extended until further notice.

Early Thursday, Palestinian militants in the Gaza Strip fired at least five mortar shells and one Qassam rocket at the western Negev, Army Radio reported. The barrage came after Hamas threatened to avenge the shooting deaths of four of its members a day earlier on the Israel-Gaza border.

The incident began when soldiers from the IDF Paratroopers Brigade spotted the group of Palestinians moving toward the border fence near the central Gaza town of Khan Yunis.

The troops subsequently opened fire on the militants, whom the IDF said were trying to lay an explosive device near the fence.

Duing an ensuing gunbattle, a number of mortar shells were fired at the IDF troops and an explosive device was detonated, according to the army.

One IDF soldier sustained a light wound to his hand, and was taken for treatment to the Soroka Medical Center in Be'er Sheva.

Palestinian Health Ministry official Dr. Moawiya Hassanain said ambulances later retrieved the bodies of four Hamas militants from the area of fighting. They were identified as Mahmoud Siam, Rami Farina, Mohsen al-Kudra and Ismail Abu al-Allah.

The IDF said various weapons and grenades were found on their bodies.

Hamas' military wing threatened retaliation, and within hours of the incident on Wednesday, at least six shells and two rockets exploded in the Negev. There were no injuries or damages in any of the incidents.

"The anger of our people and our resistance will reach everybody, God willing, and our response to the enemy will be painful, and will spill the Zionists' blood," the wing's spokesman, Abu Obeida, said in a statement following the gunbattle.

IDF turns to new 'security parameter' tactics

Defense sources told Haaretz on Wednesday that the army is now authorized to act within what the IDF calls the "security parameter" - a several-hundred-meter strip beyond the border fence. The troops are allowed to act only against clear threats and with the approval of a senior commander.

The fragile five-month truce began to unravel on November 4 when IDF paratroopers killed six Hamas gunmen after discovering a tunnel close to the Kisufim roadblock. Since then IDF forces have crossed the border at least twice to dismantle explosives or strike militants who tried to plant bombs near the fence.

Until last week's operation, the IDF refrained from offensive action to neutralize threats.