Thousands of Hamas supporters have attended a rally in Gaza to honour assassinated commander Mahmoud al-Mabhouh.
The group has blamed his death last month in a luxury hotel in Dubai on an Israeli hit squad.
"The decision to avenge the martyr Mahmoud al-Mabhouh has been taken, and it will be equal to the crime," Abu Obeida, a spokesman for Hamas's armed wing, told the crowd at Wednesday evening's rally in Gaza.
"All you killers can do now is wait," he said.
"We are the ones who will decide the tools suitable to carry out our promise. We will not tell you how or where or when, but only to prepare to receive the hellfire of our anger."
The rally was attended by members of al-Mabhouh's family as well as senior members of Hamas and its military wing, some of whom trampled on a long Israeli flag laid on the ground.
Shortly after the speech exiled Hamas leader Khaled Meshaal gave a televised address from Damascus in which he again blamed the killing on Israel's Mossad spy agency.
"The time for promises and talk of revenge is done. Now is the time for action," he said.
Mossad, the Israeli spy agency, had made no comment on claims that it was behind the killing, but earlier on Wednesday Israel's foreign minister, Avigdor Lieberman, told Israeli army radio there was "no reason" to believe that the agency was involved.
However, he also said that Israel has a "policy of ambiguity" on intelligence matters.
Dubai police have named 11 suspects in the killing, all of whom entered the emirate on forged passports, and investigators have refused to rule out that they were working for Mossad.
But Theodore Karasik, director of research at the Institute for Near East and Gulf Military Analysis in Dubai, told Al Jazeera the operation appeared "too sloppy" to have been carried out by Mossad.
"They were clearly unprofessional ... in the sense that they were able to go out and come to Dubai knowing full well of the biometric system that's in place here and the passport control."
Dubai officials have said that at least seven of the 11 members of the gang suspected of carrying out the killing of Mahmoud al-Mabhouh share names with foreign-born Israelis.
|At least seven of the 11 suspects are said to share names with foreign-born Israelis [AFP]
Six of the men are Britons who immigrated to Israel. The seventh is an American-Israeli, whose name Dubai said was on a German passport used by one of the assassins.
But the Israelis have insisted that their identities were stolen and said the passport pictures were not a match.
Paul John Keally, an Israeli-British citizen whose identity was apparently used by one of the group, said his life has become "like an espionage movie".
"It is all very worrying but I know I have not done anything wrong," he told Britain's Daily Mail newspaper.
The wife of Stephen Hodes, another British-Israeli living west of Jerusalem, told Israeli paper Maariv: "It started like a story that made us laugh, but now we don't know how to take it."
On Tuesday, British and Irish officials said that they had examined the details of the nine suspects' passports purportedly issued by the two countries and said they believed they were fake.
"I think Britain recognises that Israel is a responsible country and that our security activity is conducted according to very clear, cautious and responsible rules of the game"
Israeli foreign minister
Claims that Mossad may have counterfeited UK passports to allow Israeli agents to operate abroad triggered speculation that diplomatic relations between Britain and Israel could be harmed, but Lieberman said that was unlikely.
"I think Britain recognises that Israel is a responsible country and that our security activity is conducted according to very clear, cautious and responsible rules of the game," he said.
"Therefore we have no cause for concern."
Gordon Brown, Britain's prime minister, has called for an investigation into how fraudulent UK passports were apparently used in the operation and Israel's ambassador to London has been summoned to the foreign ministry.
"The defrauding of British passports is a very serious issue. The government will continue to take all the action that is necessary to protect British nationals from identity fraud," a British government spokesman said.
Mossad hit squads have used foreign passports in the past, most notably in 1997 when agents entered Jordan on Canadian passports and bungled an attempt to kill Khaled Meshaal, the exiled Hamas leader, with poison.
Britain has previously protested to Israel about what London called the misuse by Israeli authorities of forged British passports and said it received assurances steps had been taken to prevent future occurrences.
French authorities have said a national passport used by one of the suspects had a valid number but incorrect name, while Austria has launched an investigation into the suspected use of at least seven mobile phones with pre-paid Austrian chips.
The group of suspects were linked together through videos which show them entering and exiting the hotel and going in and out of the elevator on the floor where al-Mabhouh was staying.
Al-Mabhouh was born in the Gaza Strip, but had been living in Syria since 1989.
He is said to have engineered the capture of two Israeli soldiers during a Palestinian uprising in the 1980s.