The Israeli military said on Monday it had begun carrying out strikes on Hamas targets in the Gaza Strip, hours after a Palestinian rocket struck a house near Tel Aviv.
Reuters witnesses heard explosions in Gaza just before the military put out its statement.
The military said in a statement that it had "begun striking Hamas targets throughout the Gaza Strip."
One position hit was a Hamas naval position west of Gaza City, and a another was a large Hamas training camp in northern Gaza, Palestinian security officials and Hamas media outlets said.
Both positions were likely to have been evacuated, as Hamas had hours of notice that Israeli strikes were coming.
Witnesses said three missiles hit the northern target.
A rocket from Gaza hit a house in a rare strike north of Tel Aviv on Monday, wounding seven Israelis and leading Prime Minister Benjamin Netanyahu to cut short a Washington trip while vowing a forceful response.
The rocket from the Palestinian enclave and expected Israeli response come at a highly sensitive time just ahead of Israel s April 9 elections.
Israel s army said the rocket was fired by Hamas, the Islamist movement that runs the Gaza Strip, from the Rafah area in the south of the territory.
A Hamas official, speaking to AFP on condition of anonymity, denied the group was behind the rocket, evoking the possibility it was caused by "bad weather".
Israel however made it clear it was preparing a firm response, announcing it was sending two additional brigades to reinforce the Gaza area and carrying out a limited call up of reservists.
Israeli roads near the Gaza Strip were closed and farming activities in the area were halted.
Israel also closed its people and goods crossings with the blockaded Gaza Strip and reduced the zone in the Mediterranean it allows for Palestinian fishermen off the enclave, a statement said.