The Israeli military said Hamas had fired 47 rockets since it suspended strikes in Gaza and, as a result, Israel had resumed its operation after a six-hour break.
Under a blueprint announced by Egypt, Gaza's neighbour and whose military-backed government has been at odds with Hamas Islamists, a mutual "de-escalation" of week-old fighting was to have begun at 6am yesterday, with hostilities ceasing within 12 hours.
Hamas' armed wing, the Izz el-Deen al-Qassam Brigades, rejected the ceasefire, saying its battle with Israel would "increase in ferocity and intensity".
But Moussa Abu Marzouk, a top Hamas official who was in Cairo, had said the movement, which is seeking a deal that would ease border restrictions imposed by both Egypt and Israel, had made no final decision on the proposal.
Israel's Iron Dome anti-missile system intercepted several rockets over the port city of Ashdod, where a factory was hit. Emergency services said no one was hurt.
Sirens also sounded in areas up to 80 miles north of the Gaza Strip. The Qassam Brigades claimed responsibility for some of the rocket launchings.
Gaza health officials said at least 184 Palestinians, most of them civilians, have been killed in eight days of fighting, the worst Israel-Palestinian flare-up in two years.
Prime Minister Benjamin Netanyahu, whose security cabinet voted 6-2 earlier on Tuesday to accept the truce, had cautioned that Israel would respond strongly if rockets continued to fly.
An Israeli official, speaking as the Israeli strikes resumed, said: "The prime minister and the defence minister have ordered the Israeli armed forces to take powerful action against terrorist targets in Gaza."
Earlier, Sami Abu Zuhri, a Hamas spokesman in Gaza, said that demands the movement has made must be met before it lays down its weapons.
Other Palestinian militant groups — Islamic Jihad, the Popular Front for the Liberation of Palestine and the Democratic Front for the Liberation of Palestine — also said they had not yet agreed to the Egyptian offer.
Palestinian President Mahmoud Abbas, who reached an agreement with Hamas in April that led to the formation of a unity government last month, urged acceptance of the proposal.
Hundreds of rockets fired at Israel have caused no fatalities, largely due to Iron Dome, but the strikes are a threat that send people rushing into shelters.
Israel had mobilised tens of thousands of troops for a threatened Gaza invasion if the rocket salvoes persisted.
In overnight attacks before the brief ceasefire, Israel said it had bombed 25 sites in Gaza. Palestinian medical officials said a 63-year-old man and a 52-year-old woman were killed
Under the ceasefire proposal announced by Egypt's Foreign Ministry, high-level delegations from Israel and the Palestinian factions would hold separate talks in Cairo within 48 hours to consolidate the ceasefire with "confidence-building measures".
Hamas leaders have said a ceasefire must include an end to Israel's blockade of Gaza.