At least 12 people were killed, including women and children, as Israel launched a series of airstrikes in the blockaded Gaza Strip early Tuesday.
Palestinian Health Ministry confirmed the presence of women and children among the dead but did not immediately give further details on the identity of the victims.
An Agence France-Presse (AFP) journalist in Gaza saw the top of a building on fire immediately after the strikes as well as ambulances evacuating victims.
The Israeli army claimed it had targeted three leaders of the Islamic Jihad, a Palestinian armed group and hit "weapon manufacturing sites" belonging to the group.
Army spokesperson Richard Hecht told journalists the force "achieved what we wanted to achieve" in the overnight strikes, which he said involved 40 aircraft.
The armed group confirmed three of its senior officials were killed, naming them in a statement as Jihad Ghannam, secretary of the Al-Quds Brigades military council, and Khalil al-Bahtini, also of the council and commander of the military wing in northern Gaza.
The third, Tareq Ezzedine, was described by Islamic Jihad as "one of the heads of military action" in the occupied West Bank who operated from Gaza.
In Rafah, in the south of the Gaza Strip, an AFP photographer saw the body of a man identified as Ghannam.
"We mourn the leaders and their wives and a number of their children who were killed in a cowardly Zionist crime," Islamic Jihad said in a statement, vowing "the blood of martyrs will increase (the) resolve" of the movement.
An AFP photographer saw a boy's body in the morgue of Gaza City's Shifa hospital, where mourners had gathered.
Asked by journalists about child casualties, Hecht said, "If there were some tragic deaths we'll look into it."
Explosions from air strikes, which began a little after 2 a.m. (11 p.m. GMT), could be heard for nearly two hours, according to AFP journalists.
The operation came less than a week after Islamic Jihad announced a truce around Gaza – brokered with help from Egypt – following a fresh flare-up in violence.
Israel and Gazan armed groups traded cross-border fire following the death in Israeli detention of Khader Adnan, who had been on hunger strike for 87 days following his arrest over ties to Islamic Jihad.
On Tuesday, the armed group said Israel had "scorned all the initiatives of mediators" and vowed it would "avenge the leaders" killed in the latest airstrikes.
In separate statements detailing each of the Islamic Jihad figures killed, the Israeli army affirmed it would "continue to operate for the security of the civilians in the state of Israel."
The military presented Ghannan as "one of the most senior members of the organization" who was entrusted with coordinating weapons and money transfers between Hamas and Islamic Jihad.
Bahtini was "responsible for the rocket fire toward Israel in the past month," Israel said.
Ezzedine was recently "planning and direction (sic) multiple attacks against Israeli" civilians in the West Bank, where he was from, and which has been occupied by Israel since the 1967 Six-Day War.
He was sentenced to 25 years in prison by Israel for his involvement in suicide attacks in the 2000s, before being freed in a 2011 prisoner exchange and transferred to Gaza, according to the army.
Hecht said the military was "looking where this thing will develop", with the army instructing Israeli residents within 40 kilometers (25 miles) of the Gaza border to stay near bomb shelters until Wednesday evening.
In a statement, Hamas chief Ismail Haniyeh said, "Assassinating the leadership in a treacherous operation will not bring security to the occupier, but instead greater resistance."
The armed group's spokesman, Hazem Qassem, warned that Israel "bears responsibility for the repercussions of this escalation."
Israel and Gazan armed groups have fought multiple wars since Hamas took control of the Palestinian enclave in 2007.
A three-day conflict last August in Gaza caused 49 Palestinian fatalities and none on the Israeli side.
Tuesday's casualties bring to 120 the number of Palestinians killed in the Israeli-Palestinian conflict so far this year.
Nineteen Israelis, one Ukrainian and one Italian have been killed over the same period, according to an AFP count based on official sources from the two sides.
