Seven people, including three children,
have been killed by Israeli shells which hit a beach in the northern Gaza Strip, Palestinian officials say.
At least 30 people were wounded in the shelling, they say.
The Israeli military says it has halted all shelling of Gaza
and has launched an inquiry into whether ground-based artillery could have been involved.
In a statement, the military wing of Hamas threatened to resume attacks on Israel in the wake of "massacres".
The group has been observing a self-imposed ceasefire for more than a year.
Although there have been threats of a response to other attacks in recent months, the BBC's Simon Wilson in Jerusalem says the move is significant because it appears on the official website of the armed wing of the group.
The incidents come a day after senior Palestinian official Jamal Abu Samhadana
was killed in an Israeli air strike in Rafah, the southern Gaza.
Samhadana - the founder of the Popular Resistance Committees (PRC) -
was buried in Rafah on Friday, with thousands of mourners pledging to avenge his death.
Samhadana was one of Israel's most wanted men in Gaza, and his group has been blamed for a series of missile attacks on Israel.
Palestinian leader Mahmoud Abbas condemned the Israeli strikes in Gaza.
"What the Israeli occupation forces are doing in the Gaza Strip constitutes a war of extermination and bloody massacres against our people," Mr Abbas said in a statement carried by the Palestinian official Wafa news agency.
UK Foreign Secretary Margaret Beckett said London was "deeply concerned by reports of the deaths from Israeli
shelling of civilians, including children, on a Gaza beach".