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.
The extent of any escalation is likely to depend on whether Hamas, which controls Gaza, decides to join in.
Israel officials are said to be preparing for days of fighting.
The strikes were the deadliest since three days of hostilities between Israel and Islamic Jihad last August in which 49 Palestinians were killed in Gaza.
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.