Four Palestinian children have been killed in an Israeli air strike in the northern Gaza Strip, local medics say.
More than 200 Palestinians have been killed since November
The boys were playing near the Jabaliya refugee camp. The Israeli army said it had targeted a rocket-launching cell.
Later, two Hamas militants were killed by Israeli strikes in Beit Hanoun and on a security post near Hamas leader Ismail Haniya's home in Gaza City.
On Wednesday, a rocket fired by Hamas killed an Israeli student near Sderot, the first such death in nine months.
Israeli Prime Minister Ehud Olmert said "terrorists" in the territory would pay a very heavy price for the attacks.
The US Secretary of State, Condoleezza Rice, said that the Palestinian rocket attacks "need to stop".
Ms Rice will visit the Middle East next week for talks with Mr Olmert and Palestinian Authority President Mahmoud Abbas.
UN Secretary General Ban Ki-moon has separately expressed "deep concern" at the escalation of violence in which at least 24 Palestinians have been killed in the last two days.
A string of Israeli air raids targeted Hamas militants in the Gaza Strip on Thursday.
In the afternoon, four boys reportedly aged between eight and 15 were killed whilst playing football near the al-Salam mosque in Jabaliya. Three of them were members of the same family, according to the Palestinian news agency Maan.
GAZA AIR STRIKES
Four children killed near Jabaliya refugee camp
Hamas militant killed near Shati refugee camp
Hamas militant killed near Beit Hanoun
Three Hamas militants and two from Popular Resistance Committees killed in Gaza City
Six-month-old boy killed near interior ministry
Five Hamas militants near Khan Younis
Islamic Jihad militant near Bureij refugee camp
An Israeli army spokeswoman told the AFP news agency it had launched "several strikes that targeted and hit rocket-launching cells" in the area.
Shortly afterwards, a separate Israeli strike in nearby Beit Hanoun killed a member of Hamas' military wing, Izzedine al-Qassam Brigades, medics said.
A Hamas security post 150m from the home of the former Palestinian Prime Minister, Ismail Haniya, was also attacked by an Israeli helicopter, killing a member of Hamas and wounding four other people, officials said.
Hamas said Mr Haniya had not been in the area at the time and his home was not damaged. After his office was bombed on Wednesday, Mr Haniya condemned what he called "continuous crimes" by Israel.
Earlier on Thursday, Hamza al-Haya, son of a prominent hardline leader in the Gaza Strip, Khalil al-Haya, was killed along with a fellow militant. Another strike killed a Hamas militant and two from the Popular Resistance Committees.
The interior ministry building in Gaza City, run by Hamas, was hit on Wednesday evening. The building was empty.
Israel has vowed the militants will pay a heavy price
Nearby buildings were caught in the blast, killing a baby and wounding at about 30 other people, Palestinian medical officials said.
Most of the other recent Palestinian fatalities have been militants, including five Hamas members killed in an air strike in Khan Younis on Wednesday morning. Another two were killed by Israeli troops in the Balata refugee camp.
One of Hamas's rockets exploded in a car park at Sapir College in Sderot late on Wednesday afternoon, killing an Israeli 47-year-old studying at the college.
Although the Israeli fatality was the first caused by rocket fire in nine months, four other Israelis have been killed in attacks by Palestinian militants since the relaunching of peace talks between Israel and the Palestinian Authority.
In that time, more than 200 Palestinians have been killed by the Israeli military.