Israel's prime minister has said his country is at war as it retaliates against the attacks by the Islamist militant group Hamas. More than 400 people on both the Israeli and Palestinian sides are feared dead, and there are growing concerns that the situation will worsen.

Hamas on Saturday launched a wave of rocket attacks and its fighters crossed into Israel from the Gaza Strip.

The Israeli military says that more than 3,000 rocket rounds were launched, killing 200 Israelis and injuring more than 1,000. It says a number soldiers and citizens have been taken hostage.

Hamas said in a statement that it had to make a strategic decision to counter what it called the crimes of Israel against the Palestinian people.

Israeli Prime Minister Benjamin Netanyahu said that his country is at war. Israeli forces are conducting air raids on Hamas targets in Gaza.

He said that "all of the places which Hamas is deployed, hiding and operating in" will be turned "into an island of ruins." He told the people of the Gaza Strip to leave immediately.

Health authorities in Gaza said that 232 people there were killed and nearly 1,700 were injured.

A spokesperson for UN Secretary-General Antonio Guterres released a statement, saying the UN chief condemns the attack in the strongest terms. The statement says Guterres is appalled by reports that civilians have been attacked and abducted.

It says violence cannot provide a solution to the conflict and that the UN chief urges all diplomatic efforts to avoid a wider conflagration.