At least 13 Palestinians, including three commanders of the militant group Islamic Jihad, have been killed in Israeli air strikes on the Gaza Strip.

Palestinian health officials said eight women and children were among the dead. Another 20 people were injured.

Israel said it had launched an operation targeting militants who posed an imminent threat to its citizens.

Islamic Jihad has vowed revenge and Gaza-based militants are expected to respond with rocket fire into Israel.

Israel officials are said to be preparing for days of fighting.

The strikes were the deadliest since three days of fighting between Israel and Islamic Jihad last August.

The latest strikes took place in the early hours of Tuesday morning, when 40 Israeli warplanes and helicopters attacked in several waves across Gaza, hitting homes and causing panic among residents.

Pictures showed at least two apartments with their fronts ripped away and others damaged.

The Gaza health ministry said four children and four women were among those killed. Half of the injured were women and children and several were in a critical condition in hospital, it added.

Russia's representative office in the Palestinian territories announced that one of its citizens, Dr Jamal Khuswan, a former chairman of the Gaza Dentists' Association, was killed along with his wife and their son.

Another two of their children survived, it said. They included 10-year-old Diala, who was filmed sitting in the front seat of an ambulance and crying out for her dead father.

Mr Khuswan's brother, Mohammed, said he was a very-well known dentist and that the family had been at their home when it was hit by a missile.

Palestinian sources said they lived next to one of the senior militants who was killed.

Islamic Jihad's military wing, the al-Quds Brigades, confirmed the deaths of three of its commanders, along with their wives and a number of their children.

It identified them as Jihad Shaker al-Ghannam, secretary of the al-Quds Brigades' Military Council; Khalil Salah al-Bahtini, the commander of its Northern Region, and Tariq Mohammed Ezzedine, who it said was "one of the heads of military action" in the occupied West Bank.

"We affirm that the blood of the martyrs will increase our resolve," the al-Quds Brigades said. "We will not leave our positions, and the resistance will continue, God willing."

Israel's Defence Minister, Yoav Gallant, said: "Any terrorist who harms Israeli citizens will be made to regret it."

The Israel Defense Forces (IDF) said Bahtini was the senior operational officer of Islamic Jihad in Gaza, and that he was responsible for the rocket fire from Gaza in the last month.

"He was an imminent threat to the security of Israeli civilians."

It added that Ghannam was a senior member of Islamic Jihad's rocket force and that Ezzedine was in charge of co-ordination with the group's operatives in the West Bank and planning attacks on Israeli civilians there.

The IDF said its aircraft also struck 10 sites used to manufacture weapons and six Islamic Jihad military facilities.

Spokesman Lt Col Richard Hecht said the operation had "achieved what we wanted to achieve", according to AFP news agency.

When asked about child casualties, he replied: "If there were some tragic deaths, we'll look into it."

He also said that residents of Israeli communities within 40km (25 miles) of Gaza had been advised to stay close to bomb shelters until Wednesday evening.

The leader of Hamas, the Palestinian militant group which controls the Gaza Strip, Ismail Haniyeh, warned: "Assassination of leaders will not bring the Occupation [Israel] security but more resistance."

"The enemy mistake in its calculations and will pay the price for its crime," he added.

Correspondents say one significant factor in determining the scale of any retaliation will be the extent to which Hamas joins in.

The spokesman for West Bank-based Palestinian President Mahmoud Abbas, Nabil Abu Rudeineh, said the Israeli government "bears full responsibility for this dangerous escalation".

Officially known as the Palestinian Islamic Jihad (PIJ), it is the biggest militant group in Gaza next to Hamas. It has been responsible for many of the rocket attacks Israel from the territory in recent years and is sworn to Israel's destruction.

There was a serious flare-up last week, as Islamic Jihad and other groups fired several barrages of rockets into Israel over two days following the death in an Israeli prison of a Palestinian hunger striker. The Israeli military carried out air strikes in response.