No injuries were reported as a result of the Palestinian projectiles, at least 60 of which fell on Israeli towns and cities by nightfall on Wednesday.
These two rockets hit a house and an amusement park in Ashkelon, an Israeli city 13km south of Gaza, Israeli and Palestinian officials said.
Hamas and Islamic Jihad claimed responsibility for the barrage, which they said were in response to Israel's killing of three Palestinian fighters the previous day.
Ban Ki-moon, the UN secretary-general, condemned the rocket fire and called on Hamas to ensure they ceased immediately.
"[Ban] urges all parties to work to secure the immediate restoration of the calm and an urgent easing of humanitarian conditions in the Gaza Strip," a statement from his spokesperson said.
Matan Vilnai, Israeli deputy defence minister, said on army radio that the attacks were "intolerable and we will take all necessary measures to stop them".
Two of the rockets were Grads, a kind of rockets with a longer range and better precision than the home-made projectiles normally used by Palestinian fighters.
After the ceasefire between Israel and Hamas ended on Friday, Israel had threatened to launch a major offensive on Gaza while Hamas said it would retaliate by resuming suicide attacks inside Israel.
Tzipi Livni, the Israeli foreign minister, is scheduled to meet Hosni Mubarak, the Egyptian president, on Thursday for talks on a possible renewal of a truce.
Israeli media reported on Wednesday that Israel would want to tie any future truce agreement to the fate of an Israeli soldier kidnapped by Hamas in 2006 and held in Gaza.
In response to the latest rocket fire, Israel said it would keep Gaza sealed on Wednesday, cancelling plans made earlier to open crossings to allow aid deliveries.
Al Jazeera's correspondent Sherine Tadros, reporting from Gaza, said UN food distribution centres are closed due to shortage of supplies.
She said "750,000 Palestinian refugees rely on the UN food aid to survive and they are not able to get that aid for at least another week.
"The other main problem is the fuel situation. Seventy per cent of Gazans are affected by rolling blackouts because there isn't enough fuel for the main power plant to start up again.
"As the security situation is worsening, we see humanitarian disaster in Gaza."