Hamas said Mr Mabhouh was behind the abduction of two Israeli soldiers
A senior Hamas military commander has been assassinated by Israel in Dubai, the Palestinian Islamist group claims.
Mahmoud al-Mabhouh, 50, a founder of the Izzedine al-Qassam Brigades, "died a martyr on 20 January in suspicious circumstances", a statement said.
Hamas gave no further details, but vowed to "retaliate for this Zionist crime at the appropriate moment".
An Israeli government spokesman would not comment, in line with Israel's usual policy on similar allegations.
The AFP news agency reports that the authorities in Dubai have identified several "European passport holders" as suspects.
"Preliminary investigations indicate that the crime was committed by a professional criminal gang that was following the victim before he came to the United Arab Emirates" it quoted a statement as saying.
The members of the gang managed to flee the UAE before the death was reported, the statement added.
'Close to leader'
A Hamas political bureau member in Damascus, Izzat al-Rishq, told the BBC that Mr Mabhouh, who had been living in Syria since 1989, had been very close to its exiled political leader, Khaled Meshaal.
Hundreds of people took to the streets as Mr Mabhouh's coffin was paraded
"Mabhouh died a martyr in Dubai on 20 January 2010 in suspicious circumstances that require an inquiry in co-operation with the United Arab Emirates authorities," Hamas said in a statement.
"We in Hamas hold the Zionist enemy responsible for the criminal assassination of our brother, and we pledge to God and to the blood of the martyrs and to our people to continue his path of jihad and martyrdom," it added.
Hamas has not given details of how he was killed, but Mr Mabhouh's family said medical teams that examined him determined that he had died in his hotel room after receiving a massive electric shock to the head. They also found evidence that he had been strangled.
Blood samples sent to a French laboratory confirmed he was killed by electric shock, after which the body was sent to Syria, they said.
Thousands of people attended Mr Mabhouh's funeral at the Yarmouk Palestinian refugee camp, on the outskirts of Damascus, on Friday.
In a speech, Mr Meshaal described him as a "great man" who had fought Israelis for 30 years.
"I say to you Zionists, do not rejoice. You killed him but his sons will fight you," he said.
"God already took our leaders and loved ones, but resistance goes on. Palestine is a blessed land. It will not remain patient."
Hamas said Mahmoud al-Mabhouh had been responsible for the abduction in 1989 of two Israeli soldiers, who were both later killed.
Sgt Avi Sasportas and Sgt Ilan Sa'adon were seized a few months apart as they hitchhiked from military bases to their homes during the first Palestinian Intifada.
Our brother had been a target for the occupier ever since his participation in the kidnapping operation against the two Zionist soldiers
Sgt Sasportas's body was discovered a year later, close to where he was picked up as he went home to Ashdod. Sgt Sa'adon's body was not recovered for seven years.
Mr Mabhouh also masterminded a number of other attacks, for which the Israeli authorities demolished his home in Gaza, Hamas said.
He spent several periods in Israeli custody. After his last release, "he spent his life being hounded by the Zionist occupier until he succeeded in leaving the Gaza Strip," it said.
"Our brother had been a target for the occupier ever since his participation in the kidnapping operation against the two Zionist soldiers, and for his role and support for the resistance."
History of assassinations
The Izzedine al-Qassam Brigades have carried out hundreds of attacks and suicide bombings targeting Israeli troops and civilians.
These include launching rockets from the Gaza Strip into Israeli territory, the main reason the Israeli military gave for launching a 22-day offensive on Gaza in December 2008, which left about 1,400 Palestinians and 13 Israelis dead.
Hamas's charter effectively calls for the destruction of Israel, although its leaders have more recently said they would consider a long-term ceasefire in exchange for a state on the land Israel occupied in 1967.
Israel has a long history of assassination operations targeting militants. Most famously, in 1987 in Tunisia, agents killed Abu Jihad, the Palestine Liberation Organisation's military leader.
But in 1997, one mission went wrong, when two agents were arrested in Jordan after attempting to poison Mr Meshaal and Israel was forced to hand over an antidote by the US government.
More recently, Israel denied that it was behind the assassination of Imad Mughniyeh, the military commander of the Lebanese Shia militant group, Hezbollah, in Damascus in 2008.
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