"Israeli planes and troops on the ground are carrying out an attack in the Gaza Strip," the army said in a brief message.

Army spokesman Jonathan Conricus confirmed that Israeli soldiers had entered the Palestinian enclave, without giving any numbers.

Earlier Thursday, Israel pounded Gaza and deployed extra troops and tanks to the border as Palestinians fired barrages of rockets back, with the death toll in the enclave on the fourth day of conflict climbing to over 100.

"We are ready and we will continue to prepare ourselves for various scenarios," the army spokesman said earlier, specifying that a ground invasion was "one of the scenarios".

The Israeli defence ministry has given the army the green light to mobilise thousands of reservists if necessary.

"Any ground incursion in any part of the Gaza Strip will lead to an increase in the number of dead and prisoners from the enemy side," warned the armed branch of Hamas which continued to launch rockets towards southern Israel.