An Israeli air strike killed a Palestinian militant and wounded another in the Gaza Strip, witnesses and their organisation said Wednesday.
The missile attack on a vehicle in Rafah near the border with Egypt posed a new threat to a truce announced late Sunday following four days of clashes between Israeli forces and Palestinian militants.
The armed branch of the radical Islamic Jihad movement, the Al-Qods Brigade, said the dead man was one of its leaders, Ismael al-Ismar, 34.
An Egyptian-brokered halt to Palestinian rocket fire from the Gaza Strip had appeared to be holding on Tuesday, with the Israeli military saying calm had prevailed along the border overnight.
The truce was announced by a senior official in Hamas, which has ruled the Gaza Strip since 2007, following four days of violence sparked by ambushes near Eilat in southern Israel on Thursday in which eight Israelis died.
Gaza's Popular Resistance Committees (PRC), a grouping of militant organisations, said it would join the truce a day later.
Although four rockets were fired into southern Israel in the following hours, Israel did not respond, with the press attributing it to "small terror groups looking to challenge Hamas and demonstrate their independence."
Defence Minister Ehud Barak, meanwhile, said in televised remarks: "This is a delicate situation and there is a real risk of endangering the (1979 Egyptian-Israeli) peace treaty, which is a precious strategic asset for Israel."
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"The whole Middle East is a powder keg and an Israeli action could have consequences for what is happening in Egypt, in Syria and in Libya," army radio cited a high-ranking military official who spoke on condition of anonymity as saying.
The Hamas official for his part had warned that the truce depended on Israel respecting it.
The latest unrest was sparked by a series of shooting ambushes near Eilat on Thursday which killed eight Israelis and was blamed on the PRC.
In the following days, Israeli air strikes killed 15 Palestinians, 12 of whom the military says were militants, and more than 50 people were wounded. Among those killed was PRC chief Kamal al-Nayrab, who died in an Israeli air strike on Rafah, in the south of Gaza.
Palestinians fired more than 100 rockets and mortar shells at Israeli towns and cities in the south, killing one man and injuring more than 20, one critically.
Israel on Tuesday sent a complaint to UN Secretary General Ban Ki-moon, objecting to the lack of Security Council condemnation of the Eilat shooting, according to the foreign ministry.
Lebanon, which currently holds a Security Council seat, on Friday blocked a statement which would have called the deadly attacks in southern Israel terrorism.
The move brought criticism from the United States which said the terrorism label is a "standard" Security Council description after such an attack.
The last ceasefire was agreed on April 10, after another cycle of violence that began when an anti-tank missile slammed into an Israeli school bus.
During that flareup, 18 Palestinians were killed, half of them civilians, and more than 150 rockets and mortar rounds fired into southern Israel.