Israel's navy has shot and killed four Palestinians wearing diving gear off the Gaza coast, officials say.
The Israeli military said it believed those on board the boat were planning a terrorist attack.
Hamas officials in Gaza say four bodies have been recovered and two people are missing.
It comes a week after nine pro-Palestinian activists died in an Israeli raid on an aid flotilla trying to break Israel's blockade of Gaza.
Israel withdrew its forces from Gaza in 2005, but still controls the sea off the territory's coast.
The Israeli military said the boat was carrying "a squad of terrorists wearing diving suits on their way to execute a terror attack".
A spokesman said a naval force had hit its target, but did not give any more details of the operation.
The country's Haaretz newspaper quoted an Israeli army source as saying the incident took place at about 0430 local time (0130 GMT), and that the boat had been heading north to Israel from waters off the Nuseirat refugee camp in central Gaza.
The al-Aqsa Martyrs' Brigades, a militant group linked to Fatah, has reported that four of its men were killed and a fifth is missing, according to Israeli media.
The men had been training off the Gaza coast, the militant group was quoted as having claimed.
The Israeli Defence Forces (IDF) said there were no Israeli casualties.
There has been sporadic rocket fire out of Gaza into Israel since last week's naval attack on the Gaza aid flotilla.
Israeli Foreign Ministry official Yigal Palmour said the latest incident reinforced the need for Israel to maintain a tight cordon around Gaza.
"This is the explanation why the border, both land and sea border with Gaza need to be strictly and tightly controlled. We can't allow Hamas to carry out attacks at will on Israelis, on Israeli territory," he told Reuters TV.
The latest Israeli operation comes as Palestinian President Mahmoud Abbas visits Turkey for a regional security summit along with Iran and Syria.
Mr Abbas, who heads the Palestinian Authority in the West Bank, is expected to pay his respects to the nine activists killed last week, eight of whom were Turks and one a US national of Turkish origin.
Mr Abbas's government is the bitter rival of Hamas, which took control of Gaza from his Fatah movement in 2007.
The deaths have sparked global condemnation but defiance from Israel, which insists it has the right to defend itself.
Turkish Prime Minister Recep Tayyip Erdogan has accused Israel of "state terrorism", and is calling Gaza a "historic cause for Turkey".
Another aid ship, the Irish-owned Rachel Corrie, was intercepted by Israel on Saturday and officials have begun deporting its crew and activists.
France and Britain have called for Israel to accept a "credible and transparent" investigation into the deadly Israeli raid on 31 May.
French Foreign Minister Bernard Kouchner suggested that the EU could play a bigger role to ensure that humanitarian supplies reached Gaza, and that weapons were kept out.
Mr Kouchner also said Europe needed to work harder to convince Israel that its blockade of Gaza was not working, and nor was it in the long-term interests of the region.
His comments followed talks with British counterpart William Hague, who said Europe would maintain pressure on Israel.
But Israel's US ambassador Michael Oren said his country would reject the proposals, and reiterated that an internal inquiry would be held.
Meanwhile, the Iranian Red Crescent has announced it will send two aid ships to Egypt for onward delivery to Gaza through the Rafah crossing later this week, Iranian state media report.
Also on Monday, US Vice-President Joe Biden met with Egyptian President Hosni Mubarak in Sharm el-Sheikh.
They had a 90-minute meeting "about a full range of bilateral issues", a spokesman for the US Embassy said, but there were no further details of the discussions released.