The Israeli military carried out Thursday several airstrikes on the Gaza Strip in response to alleged rocket fires from the blockaded Palestinian territory.
The violence comes a day after the deadliest Israeli raid in the occupied West Bank in nearly 20 years.
Eleven Palestinians, including a 16-year-old, were killed and more than 80 wounded by gunfire Wednesday when Israeli troops raided the flashpoint West Bank city of Nablus, drawing international appeals for calm.
Top Palestinian official Hussein al Sheikh described the raid as a "massacre" and called for "international protection for our people."
Before dawn on Thursday, Palestinian armed groups reportedly hit back firing six rockets from Gaza into Israel.
The Israeli army said it managed to intercept five of them, while the sixth struck an uninhabited area.
Palestinian armed group Islamic Jihad claimed responsibility for the rockets after it called on "resistance forces" to respond to the "major crime" in Nablus.
Two hours later, the Israeli military carried out airstrikes on multiple targets in Gaza, sending plumes of black smoke into the sky.
The missiles targeted "a weapons factory" and a "military camp," both run by Gaza's Islamist rulers Hamas, the army said in a statement.
United Nations chief Antonio Guterres appealed for calm, warning that tensions in the West Bank had reached their most dangerous level in years.
"Our immediate priority must be to prevent further escalation, reduce tensions and restore calm," the U.N. chief said.
"The situation in the occupied Palestinian territory is at its most combustible in years," he added, pointing to "sky-high" tensions and the stalled peace process.
The Israeli army said Wednesday's raid had targeted a "hideout apartment" used by alleged militants accused of shootings in the West Bank.
It said one of the wanted suspects was "neutralized," along with two others at the property who had opened fire on troops.
The suspects and Israeli forces "exchanged fire ... there were also rockets that were fired on the house" by the army, spokesman Richard Hecht told journalists.
Rocks, explosive devices and Molotov cocktails were hurled at the troops, the army said, adding that nonetheless, they suffered no casualties.
The Palestinian health ministry said those killed "as a result of the occupation's aggression on Nablus" were aged between 16 and 72.
Hours after the raid, the ministry announced the death of a 66-year-old man from teargas inhalation.
Wednesday's death toll was the highest since the second Palestinian intifada, or uprising, ended in 2005, surpassing that of an Israeli raid last month in Jenin, another flashpoint city in the West Bank.
Palestinian health officials said 82 people were admitted to the hospital with gunshot wounds.
Mostafa Shaheen, a Nablus resident, said "soldiers ... besieged the whole area" at around 9:30 a.m. (7:30 a.m. GMT).
"We kept hearing the explosions and gunfire," he told AFP.
The wounded include Palestine TV journalist Mohammed al Khatib, who was shot in the hand, his colleague told AFP.
The Islamic Jihad armed group said one of its commanders was killed "in a heroic battle."
The Lions' Den, a local armed group based in Nablus, said six of those killed were members of various Palestinian factions.
Hailing the army's "courage," Defense Minister Yoav Gallant tweeted that Israel's "long arm" would reach "any terrorist."
A huge crowd of mourners, including armed men, gathered in Nablus and the nearby Balata refugee camp in the afternoon for the funerals of 10 of those killed, AFP correspondents reported.
The Palestinian Red Crescent Society said its medics had treated 250 cases of tear gas inhalation and dozens of gunshot wounds.
State Department spokesman Ned Price said Washington was "extremely concerned by the levels of violence," while EU foreign policy chief Josep Borrell said the European Union was "deeply alarmed by the spiraling violence in the West Bank."
Borrell called on "all parties" to work towards "restoring calm and ... avoid further loss of life."
Condemning the acts of violence committed against civilians, France reiterated Israel's obligation to respect international humanitarian law and use proportionate force.
Neighboring Jordan said it would "work intensely with all parties to achieve" calm.
Since the start of this year, the Israeli-Palestinian conflict has claimed the lives of 60 Palestinian adults and children, including armed attackers and civilians.
Nine Israeli civilians, including three children, a police officer and one Ukrainian civilian have been killed over the same period, according to an AFP tally based on official sources from both sides.
U.S. Secretary of State Antony Blinken spoke Saturday with Israeli Prime Minister Benjamin Netanyahu and separately with Palestinian president Mahmud Abbas, calling on both to "restore calm."
Israel has occupied the West Bank since the Six-Day War of 1967.
Last year was the deadliest year in the territory since the United Nations started tracking casualties in 2005.
