Violence has flared between Israel and Palestinian militants in Gaza, a day after seven militants and an Israeli soldier were killed during an undercover Israeli operation in Gaza.
Militants fired 300 rockets and mortars at Israel. One hit a bus, seriously injuring a soldier nearby.
Israel responded with more than 70 strikes on what it said were targets belonging to Hamas and Islamic Jihad.
Three Palestinians, two of them reportedly militants, were killed.
The Hamas-run health ministry in Gaza said nine Palestinians were injured.
Meanwhile, Israeli medics said 10 people in Israel were injured.
Israeli media later reported that a man was killed after a house was hit by a rocket in the Israeli city of Ashkelon.
Israeli Maj Gen Kamil Abu Rukun warned that Hamas "has crossed a red line" and Israel would "continue to respond with an iron fist against all terrorist activity or rocket fire"".
Israeli Prime Minister Benjamin Netanyahu cut short his visit to France for an urgent meeting with his security chiefs.
What happened on Sunday?
A Hamas commander and an Israeli soldier were among the dead on Sunday.
Palestinians said an Israeli unit travelling in a civilian vehicle had killed the Hamas commander.
According to Palestinian sources, the Israeli unit was about 3km (2 miles) inside the Gaza Strip, which borders Israel, when militants from Hamas, which controls the Gaza Strip, stopped the car.
The group's military wing, the Izzedine al-Qassam Brigades, said the Israelis opened fire, killing a local commander, Nur Barakeh.
The incident is reported to have happened east of Khan Younis, in the south of the territory.
A gun battle erupted and Israeli tanks and aircraft opened fire in the area, witnesses said.
Six of the Palestinians killed belonged to Hamas and the seventh was a member of the militant Popular Resistance Committees, AFP news agency cited Palestinian officials as saying.
The Israel Defense Forces (IDF) said a member of the special unit involved was killed and another was lightly wounded.
The Israeli soldiers were later flown back by helicopter.
Why did Israel kill the commander?
Due to the secrecy of the operation, Israel has not revealed specific details about the mission.
The IDF said, however, that the operation was "not intended to kill or abduct terrorists, but to strengthen Israeli security".
The BBC's Tom Bateman in Jerusalem says that according to a former Israeli general, the incident was likely to have been an intelligence-gathering operation that went wrong.
The exposure of such an operation by Israeli special forces inside Gaza would be extremely rare, he says.
What has happened since Sunday's operation?
Fawzi Barhoum, a spokesman for Hamas, denounced the incident as a "cowardly Israeli attack".
IDF chief Lt Gen Gadi Eisenkot said the Israeli unit had carried out "a very meaningful operation to Israel's security", without giving further details.
The Israeli military said that immediately after the clashes, 17 rockets were fired from Gaza into Israel, three of which were shot down.
Throughout Monday, some 300 rockets and mortars were launched towards Israel, dozens of which were intercepted while many landed in open spaces, according to the Israeli military.
Paramedics said an Israeli soldier was seriously injured when a projectile hit a bus he was standing near in the Shaar Hanegev region.
Three people also suffered minor injuries from shrapnel in the town of Sderot.
The Israeli military said aircraft and tanks bombed more than 70 Hamas and Islamic Jihad targets in Gaza in retaliation, including militant compounds, observation posts, and rocket-launching squads.
The building that houses the Hamas-run television station in Gaza, Al-Aqsa, was also struck, Palestinian officials said. There were no immediate reports of casualties.
The Israeli military had warned that the property would be targeted, accusing the channel of broadcasting "propaganda", and staff had evacuated. Broadcasting reportedly resumed after a brief outage.
The IDF later said the military also hit Hamas's military intelligence headquarters.
The health ministry in Gaza reported that three men were killed and nine other people were wounded in the Israeli strikes. The Popular Front for the Liberation of Palestine militant group said two of the dead were its members.
UN Middle East envoy Nickolay Mladenov urged restraint from both sides.
#UN is working closely with #Egypt and all concerned to ensure that #Gaza steps back from the brink. The escalation in the past 24hrs is EXTREMELY dangerous and reckless. Rockets must STOP, restraint must be shown by all! No effort must be spared to reverse the spiral of violence— Nickolay E. MLADENOV (@nmladenov) November 12, 2018
Why are Israel and Hamas enemies?
Hamas won Palestinian elections in 2006 and reinforced its power in the Gaza Strip after ousting West Bank-based Palestinian President Mahmoud Abbas' rival Fatah faction in clashes the following year.
While Mr Abbas' umbrella Palestine Liberation Organisation (PLO) has signed peace accords with Israel, Hamas does not recognise Israel's right to exist and advocates the use of violence against it.
Israel, along with Egypt, has maintained a blockade of Gaza since about 2006 in order, they say, to stop attacks by militants.
Israel and Hamas have gone to war three times, and rocket-fire from Gaza and Israeli air strikes against militant targets are a regular occurrence.
Sunday night's incident comes after apparent progress in an Egyptian- and UN-brokered process to mediate after a series of escalations between the two sides in recent months.
More than 200 Palestinians in Gaza have been killed by Israeli forces since the end of March - most during weekly protests along the border at which thousands have expressed their support for the declared right of Palestinian refugees to return to their ancestral homes in what is now Israel.
Israel has said its soldiers have only opened fire in self-defence or on potential attackers trying to infiltrate its territory under the cover of the protests.
One Israeli soldier was killed on the Gaza-Israel border by a Palestinian sniper in July.