The Israeli military said a total of five rockets had been fired in the past 24 hours, including two that exploded prematurely inside Gaza.
Under the cease-fire, Israel pledged to halt its policy of attacking militant leaders and to ease a blockade it has imposed on Gaza since the Hamas takeover in 2007. Hamas pledged to halt rocket attacks on Israel. A number of smaller militant groups also operate in Gaza, including groups that draw inspiration from the al-Qaida global terror network.
U.N. Mideast envoy Robert Serry appealed for calm in a statement, saying he is "worried" tensions could threaten the informal truce. "It is of paramount importance to refrain from violence in this tense atmosphere and for parties to work constructively in addressing the underlying issues," he said.
Ihab Ghussein, the Hamas government spokesman, accused Israel of using the airstrikes to "divert the attention" from unrest in Israeli prisons. "They think that through escalation on Gaza front they can hide the truth," he said, and urged Egypt, the guarantor of the cease-fire, to intervene.
Palestinian prisoners have been rioting and hunger striking since a 64-year-old prisoner died of throat cancer on Tuesday. The Palestinians have blamed Israel for the man's death, saying he was not given proper medical care. The prisoner, Maysara Abu Hamdiyeh, had been serving a life sentence for his role in a foiled attempt to bomb a busy cafe in Jerusalem in 2002.
Israel's chief military spokesman, Brig. Gen. Yoav Mordechai, said the army was on alert for "riots" in the West Bank ahead of an autopsy planned later in the day. He accused the Palestinian Authority, which governs in the West Bank, of exploiting the death to "resume popular protests."