"We have people along the border [with Israel] reporting troop concentrations around Gaza," Abu Ubeida told RIA Novosti.
"We believe that the enemy is preparing a massive invasion and we warn them [the Israelis] that they will meet stiff resistance and these raids will not be a joyride for them," he said.
Israel has not yet responded to a rocket attack by Palestinian militants early Tuesday morning that injured at least 69 Israeli soldiers at the Zikim training base, located close to the border with Gaza.
The Israeli government is expected to discuss the deterioration in the situation in Gaza, and a possible reaction to the attack during a scheduled meeting on Sunday.
Ubeida said Hamas, which gained control of the Gaza Strip almost three months ago, was ready to resist an Israeli invasion.
"We have a resistance plan and we are preparing a surprise for the enemy...who will certainly suffer big losses," the Hamas spokesman said.