Israeli Defence Minister Ehud Barak on Thursday approved the call-up of 30,000 reserve soldiers, who can be called into action by the military at any point, the army's official spokesman said.

The announcement came as Israel pressed a massive air campaign against Gaza militants which threatened to expand into a ground operation after a rocket struck the sea just off the coast of Tel Aviv and a second landed just south of the sprawling coastal city.

"We are in the process of expanding the campaign," Brigadier General Yoav Mordechai told Channel 2 television as the army said militants had fired more than 380 rockets across the border.

"The defence minister approved a few minutes ago, based on the army's request, the recruitment of another 30,000 soldiers. We will determine how many of them will be called in," he said.

"This means that all options are on the table," he said.

His remarks came as the military said 274 rockets had hit the Jewish state in the 28 hours since an Israeli strike killed a top Hamas commander, and that another 112 had been intercepted in mid-flight by the Iron Dome anti-missile system.

"I can assume that tonight will be very not quiet in the Gaza Strip," Mordechai said.

Barak said the rocket fire on the central Gush Dan region was "an escalation" which would cost Gaza's militant groups dearly.

"The firing towards Gush Dan and the extent of the general (rocket) fire toward Israel is an escalation, and this escalation will have a price that the other side will have to pay," he said in a statement.

Senior cabinet minister Moshe Yaalon also warned that Israel was considering a ground operation in order to stamp out rocket fire.

"We are preparing all the military options, including the possibility that forces will be ready to enter Gaza in the event that the firing doesn't stop," he wrote in a series of postings on his official Twitter account.

"Whoever continues attacking us, his blood will be on his own head. From our perspective the operation won't stop and the army has prepared all the options in order to hit them," he said.

"The only way to convince the other side to stop the aggression is to make them pay a heavy price which will convince them to halt their fire."