Israeli warplanes have carried out three raids against the Gaza Strip after fighters pounded Israel with rockets, but no one was hurt, Palestinian security officials said.
The raids early on Wednesday targeted a training centre for the Ezzedine al-Qassam Brigades, the armed wing of Hamas at Rafah in the south of the Strip, a "workshop" in Gaza City and Hamas naval police installations in the north.
A spokesman for the Brigades said his organisation had fired 45 rockets in 24 hours at Israeli territory, a figure confirmed by Israeli police who said four border guards had been wounded, one seriously.
The rocket attacks on Israel came after six Palestinians were killed and five others injured in a series of air strikes across the coastal strip.
In a rare show of force, fighters from the armed wing of the ruling Hamas movement said they would continue firing unless the air strikes ended.
Israeli police said one of the rockets had scored a direct hit on a border police outpost in Yad Mordechai, just north of the Gaza Strip late on Monday evening.
"One border policeman was moderately injured, while another was lightly injured and six others were treated for shock," police spokesman Micky Rosenfeld told the AFP news agency.
The sharp increase in rocket fire came in the wake of a warning by the Brigades, shortly after an air strike targeted two men on a motorcycle in the central Gaza town of Deir al-Balah, injuring one of them critically.
"This is our answer to the Zionist crimes. It will continue if they carry out more strikes on Gaza," the group said in what was an unusual departure from their customary observance of a tacit truce with Israel.
On Tuesday morning, medics recovered the bodies of two Palestinian teenagers who had been killed in another early-morning air strike in Deir al-Balah.
Their deaths raised to six the number of Palestinians killed in a series of deadly Israeli strikes which began on Monday morning, just hours after gunmen from Sinai carried out an ambush along Israel's southern border with Egypt, killing an Israeli civilian.
Israel has said the sudden spike in air raids was "in no way related" to the border incident, with the military saying the air force was targeting fighters about to attack.
'Return to calm'
A Gaza Palestinian official who requested anonymity told AFP that Egypt was making efforts to restore calm between the Palestinian movements and Israel.
The official said the Palestinian movements were "ready for a return to calm as long as Israel ends its attacks".
The last major flare-up in Gaza was in March when four days of tit-for-tat violence left 26 Palestinians dead, and saw armed groups firing more than 200 rockets into Israel.
The fighting erupted when Israel assassinated a leader from the Popular Resistance Committees who it said was responsible for planning an attack along Israel's southern border with Egypt in August 2011 in which fighters sneaked over the frontier and killed eight Israelis.
Within hours, Israel hit back at Gaza, leading to eight days of bloodshed that left 26 Palestinians and an Israeli dead.