Hamas has accused Israeli agents of assassinating a veteran operative of the Palestinian militant group, saying he was electrocuted last week in a Dubai hotel room.
On Friday, Hamas' top leader, Khaled Mashaal, vowed to retaliate.
The militant group identified the man as Mahmoud al-Mabhouh, one of the founders of Hamas' military wing that has been responsible for hundreds of deadly attacks and suicide bombings targeting Israelis since the 1980s. It said he was 50 years old.
Authorities in Dubai confirmed al-Mabhouh was found dead in a hotel room a day after entering the emirate. Dubai's government said in a statement that initial investigations show the crime was likely committed by a "professional criminal gang," and that the suspects left the country before the body was discovered.
Mashaal, Hamas' exiled leader, blamed Israel for al-Mabhouh's death and pledged to strike back.
"This is an open war ... We will avenge the blood of Mahmoud al-Mabhouh," he said, standing next to al-Mabhouh's coffin during his funeral in Syria on Friday.
Israel's government had no immediate comment.
Izzat Rashaq, a top member of Hamas' exiled leadership in Damascus, told The Associated Press that details have not been released to avoid compromising an ongoing investigation, and that Hamas' delayed announcement was linked to an attempt to "reach the Israeli agents who implemented this operation."
The few details that emerged were somewhat conflicting.
Talal Nassar, an official in Hamas' media office in Damascus, said al-Mabhouh had been "poisoned and electrocuted in his hotel room in Dubai." He did not elaborate.
Al-Mabhouh's brother, Hussein, 49, who lives in the Jebaliya refugee camp in Gaza, said his brother "died by electric shock and suffocation with a piece of cloth."
Hussein al-Mabhouh added that his brother had survived two Israeli assassination attempts, including an attempt six months ago to poison him in Beirut that left him unconscious for 30 hours.
"It was expected that the Mossad would try to kill him," he told The AP, referring to the Israeli intelligence arm abroad.
Al-Mabhouh, originally from the Gaza Strip, lived in Syria and was passing through Dubai when he was killed late January 19 or early January 20, Rashaq said.
"We in Hamas hold the Zionist enemy responsible for the criminal assassination of our brother," read the statement on Hamas' website. The group pledged to "retaliate for this Zionist crime at the appropriate time and place."
Al-Mabhouh was buried later Friday at the Palestinian refugee camp of Yarmouk, near Damascus. More than 2000 Palestinians attended the funeral.
The coffin was wrapped in a green Hamas flag and a large portrait of al-Mabhouh was placed at the entrance to the mosque with the words: "Your fingerprints are everywhere ... we promise to continue in your path."
The Hamas statement said al-Mabhouh was involved in the kidnapping and killing of two Israeli soldiers in 1989 and that he was still playing a "continuous role in supporting his brothers in the resistance inside the occupied homeland" at the time of his death.
In Dubai, officials could not be immediately reached for comment.
Hamas rules Gaza but has leaders and operatives based in Syria, and elsewhere. The group's members abroad have been targeted in the past. The leader of its Damascus-based politburo, Khaled Mashaal, survived an Israeli assassination attempt in Amman, Jordan, in 1997. Last month, two Hamas men were killed in a mysterious late-night blast in Beirut. Hamas said at the time that Israel was an obvious suspect but stopped short of openly accusing Israel of the killings.
The Mossad never openly discusses its operations and Israel's government typically does not comment on incidents in which the Mossad's involvement is suspected.
It is also suspected of being behind the assassination of a senior military commander from the Lebanese guerrilla group Hezbollah in Damascus in 2008.
Dubai has for years been known as a low-risk hideaway for disgraced politicians and deposed foreign leaders but that image was shattered last March, when Chechnya's Sulim Yamadayev - a former rebel in the republic's long conflict with Russia who switched sides but then fell out with the territory's pro-Moscow leader - was shot dead in a Dubai underground parking lot.
The Emirates backs Hamas rival, the West Bank-based Palestinian President Mahmoud Abbas, but does not list the militant group as a terrorist organisation. Emirati officials have several times met with Hamas representatives in the capital Abu Dhabi.