Israeli aircraft have struck Hamas security installations in Gaza, killing at least six Palestinians, in further retaliation for attacks along the Egyptian border in which eight Israelis died.
Gaza residents said three compounds controlled by the Hamas group that rules the enclave were hit in the overnight raids on positions in Rafah.
Five of those killed were members of the Popular Resistance Committees, while the sixth was a young boy. Medical officials said that 17 others had been wounded.
The PRC confirmed to the AFP news agency that its leader had been killed, vowing that it would take revenge "against everything and everyone". The PRC spokesman also denied that the group was involved in the attacks on Thursday.
The Israeli military said that at least ten rockets had been fired from Gaza into southern Israel after the aerial bombing of Rafah. It said that two rockets fired at the city of Ashdod "caused damage and injuries at a synagogue and school", according to a statement.
An Israeli drone also hit the Zeitoun neighbourhood of Gaza city on Friday morning, but medical sources told Al Jazeera that it caused no casualties.
Israeli security forces beefed up security in the wake of the strikes, Al Jazeera correspondent Cal Perry reported from Jerusalem. He said that thousands of people were seen marching towards the mosque before midday prayers on Friday, and that hundreds of Israeli police officers had also been deployed.
Lieutenant Colonel Avital Leibovich, a spokeswoman for the Israeli military, told Al Jazeera that Israeli forces were targetting "not only specific terror organisations in Gaza, but rather anyone who has some influence ... with terror", including groups of people it suspects of launching rocket attacks.
"We do not wish to escalate. However, we are not the ones who chose to target civilian buses and vehicles, yesterday here in Israel," she said.
Ghazi Hamad, Hamas' deputy foreign minister, denied any connection between Hamas or the people of Gaza and the attacks in the Sinai.
"From the first moment we are surprised that Israel started to target people. To target civilians, to target places, to target buildings before knowing who stands behind this operation. So I think that Israel all the time they consider Gaza as a weakened point that they can target them under any circumstances," he said.
Regarding rocket attacks launched on southern Israel from Gaza, Hamad termed them "a kind of natural reaction against the Israeli aggression against our people.
"If they stop their aggression and attacks against Gaza, I think people here are interested in keeping Gaza calm and quiet," he said.
Hamad also said that Hamas was "surprised" that members of the international community had not condemned Israel for "killing civilians".
Israeli military commanders said six civilians and two soldiers were killed in a series of assaults by gunmen that targeted two buses, a car and an army vehicle on a desert road north of Israel's Red Sea resort of Eilat on Thursday.
The military said seven gunmen were killed in southern Israel, including two who blew themselves up in suicide attacks on one of the buses and in a confrontation with soldiers.
Israel said the attackers had infiltrated from the Hamas-run Gaza Strip via Egypt's Sinai desert, vowing to "hunt down" the perpetrators.
Brigadier-General Yoav Mordechai, the Israeli military's chief spokesman, told Israel Radio that the military had received intelligence reports of a possible attack before Thursday's assault, and that military deployments had been reinforced.
He said, however, that the complex desert terrain and porous nature of the border had complicated the Israeli army's mission.
Egyptians soldiers killed
Meanwhile, two Egyptian policemen were killed when the Israeli aircraft opened fire near the Rafah border town with the Palestinian Gaza Strip, the official MENA news agency quoted an Egyptian military official as saying.
"An Israeli plane was pursuing infiltrators on the other side of the border until they reached Rafah and fired at them. There were several Central Security members there and they were hit by the gunfire," the official told MENA.
Security officials said the incident took place south of Rafah, along the border with Israel.
They identified the Israeli aircraft as an Apache gunship that had been tracking the armed groups.
MENA quoted a military official as saying that troops were combing the border area and beefing up security after Thursday's attacks, which came as the military conducts a week long operation to uproot armed groups in Sinai.
In earlier attacks, Israel struck in the Gaza Strip against the Popular Resistance Committees, an armed faction that often operates independently of Hamas. The Israeli military said the PRC was behind the border attacks.
The PRC said its commander, Kamal al-Nairab, his deputy, Immad Hammad, and three other members were killed in Thursday's air strike on a home in the southern Gaza town of Rafah.
The faction vowed "double" revenge against Israel for the attack but denied responsibility for it.
"We salute [the operation] and we are proud of it, but we do not claim it," PRC spokesman Abu Mujahid told AFP at the funeral of five of the group's members on Friday.
"The occupation wants to pin this operation on us in order to escape its own internal problems."
Al Jazeera's Perry reported that another attack occurred later on Thursday in southern Israel, after the Israeli air strikes had begun.
"There has now been a fourth shooting attack near Eilat, where the earlier attacks took place," Perry said.
Perry described the Israeli retaliation on Gaza as "clearly a targeted hit".
"They are certainly ratcheting up the violence. After the evidence they say they found today it's clear they are stepping up their attacks on Gaza," he said.
Binyamin Netanyahu, the Israeli prime minister, issued a stern warning in a televised address in the aftermath of Thursday's violence.
"If the terror organisations think they can harm our citizens without a response, Israel will make them pay a very heavy price," he said.
Netanyahu continued: "If you harm Israel, we will respond immediately and very strongly," he said, noting that "those who ordered the killing of our citizens ... are no longer alive."
In Washington, Hillary Clinton, the US Secretary of State, said in a statement that the "brutal and cowardly attacks" near Eilat "appear to be premediated acts of terrorism against innocent civilians".
Ban Ki-moon, the United Nations secretary-general, also expressed concern about an "escalation" of violence in the region.
The attacks in southern Israel began around noon, when gunmen strafed a bus on route 12, a desert road which flanks the Egyptian border, about 25km north of Eilat.
Most of the windows were shattered but there were very few bullet holes in the bodywork, said an AFP reporter.
Reports said most of the passengers on the bus were Israeli soldiers on their way home from their respective bases for the weekend.
Shortly afterwards, the gunmen detonated a roadside bomb as a military vehicle rushed to the scene; they also opened fire on a second bus, killing the driver.
In another incident on route 90, a desert road near the Jordanian border, gunmen fired an RPG at cars 15km north of Eilat, security sources said. Medics said five victims died in that attack, four of whom were in a civilian car.
It was not immediately clear where the seventh victim, a soldier, was killed.
Israeli troops quickly locked down the area and engaged in a running gunbattle with the group that ended with seven of the attackers killed, Tal Russo, head of the army's southern command, told AFP.
Russo said two were shot dead in Israeli territory, while a third blew himself up with explosives strapped to his body.
Four more were killed on the Egyptian side of the border - two shot dead by Israeli troops firing across the border, and another two shot by Egyptian forces, he said.
There was no confirmation by Egyptian officials.