An Israeli air strike has killed two Palestinians in the Gaza Strip near the border, drawing the first Hamas cross-border rocket barrages in more than a year.
Palestinian medical officials said the two men killed on Tuesday were civilians.
The Israeli military said aircraft "targeted a terrorist squad identified handling an explosive device" in the central Gaza Strip near Israel's border fence.
Hamas, which controls the enclave, said its Izz el-Deen al-Qassam Brigades fired rockets into Israel in response.
The fire intensified throughout the day, with some 14 launches by mid-afternoon. No casualties were reported.
Although other armed groups have fired rockets across the border in previous surges of violence, Hamas had held its fire under unofficial truces with Israel.
Israeli air raids killed on Monday four Palestinians in Gaza, attacks which Fawzi Barhoum, a Hamas spokesman, said moved the group's military wing "to take a firm stance" and launch the rockets.
Hours before the aircraft went into action on Monday, fighters who crossed into Israel from Egypt's Sinai desert fired on Israelis building a barrier on that frontier, killing one worker, before soldiers shot dead two of the attackers.
Israel has said the sudden surge, three deadly air raids in just over 12 hours, was "in no way related" to the border incident, with the military saying the air force targeted militants planning or engaged in anti-Israeli attacks.
The overnight attack which killed the two 17-year-old youths took place shortly after midnight but their bodies were only recovered after dawn "when an ambulance could get to the scene", medics said.
Hospital staff named the pair as Mohammed Bassem Abu Mealiq and Yusef al-Talbani.
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.
(Agencies via Al Jazeera)