WASHINGTON (NYTIMES) - Counter-terrorism analysts were scrambling on Thursday (Oct 31) to try to figure out more about the new leader of the Islamic State in Iraq and Syria (ISIS) group, Ibrahim al-Hashemi al-Qurayshi, about whom almost nothing - including his real name - is publicly known.

Days after the ISIS leader, Abu Bakr al-Baghdadi, and his heir apparent were killed in back-to-back attacks by US forces in northern Syria, the group broke its silence on Thursday to confirm their deaths, announce Al-Qurayshi as the new leader and warn America: "Do not be happy".

"Nobody - and I mean nobody outside a likely very small circle within ISIS - have any idea who their new leader, 'Abu Ibrahim al-Hashimi al-Qurayshi', is," Mr Paul Cruickshank, editor of the CTC Sentinel at the Combating Terrorism Centre, said in a tweet on Thursday.

"The group has not yet released any meaningful biographical details which might allow analysts to pinpoint his identity."

Mr Daniele Raineri, a journalist and analyst who has been studying ISIS' leadership structure for more than a decade, said that the group's leaders often acquire a new nom de guerre with the appointment to a new position, meaning Al-Qurayshi may have had a completely different name last week.

The Al-Qurayshi appellation at the end of his name indicates that he is being portrayed as a descendant of the Quraysh tribe of the Prophet Muhammad, a lineage that ISIS considers to be a prerequisite for becoming a caliph or ruler of a Muslim theocracy.

Its use indicates that ISIS continues to see itself as a caliphate - even if one with practically no territory.

"It shows that while the world is ready to pronounce the Islamic State (in Iraq and Syria) dead and finished, the group's core leadership continues to believe it can operate much as it has in the past," said Mr Colin P. Clarke, a senior fellow at the Soufan Centre, a research organisation in New York.

"The suggestion is that nothing changes, allegiance should still be to the leadership, and affiliates and franchises should continue to look to Al-Qurayshi for guidance on how to operate," he said.

The announcement implied that the ISIS hierarchy had convened in order to discuss the successor question and suggested that he had been hand-picked in advance by Baghdadi. "The sheikhs of the mujahideen agreed, after consulting with their brothers and acting upon the recommendation of Abu Bakr al-Baghdadi, to pledge allegiance to the sheikh and mujahid, the scholar, doer and worshipper Abu Ibrahim al-Hashemi al-Qurayshi," the announcement said.

Ms Devorah Margolin, a senior research fellow at George Washington University's programme on extremism, said that the fact a shura, or governing council, had selected the new leader "shows the ISIS bureaucracy is still in place".

She added: "We should not underestimate them."

The announcement also called upon supporters to pledge allegiance to the new leader, a ritual that took new meaning under Baghdadi's reign, as attackers around the world recorded video pledges of fealty to the caliph before carrying out killings.

Counter-terrorism officials said to expect in the coming weeks a string of videos pledging allegiance from ISIS affiliates in Afghanistan, Sinai, the Philippines and other far-flung insurgent hot spots.

"This will possibly be followed by a message from the new leader," said Mr Laith Alkouri of the business risk consulting company Flashpoint Global Partners, who monitors the group's online messages.

"He's inherited the burden of leading the group and he needs to put on a show to maintain the morale of the support base."

Mr Clarke added, "This selection of the new leader is a way for ISIS to link its past with the future it will promise its adherents and followers."

Intelligence officials and analysts were trying to make sense of the announcement and to place the announced successor among the known cadre of ISIS.

Some speculated that Abu Ibrahim al-Hashemi may be a new nom de guerre of Hajji Abdullah, who appears in recently recovered internal ISIS records archived by Aymenn al-Tamimi, a researcher.

"It could be to create confusion or for operational security reasons," said Dr Aaron Zelin, a fellow at the Washington Institute for Near East Policy.

The recent announcement by the State Department of a US$5 million (S$6.8 million) reward for information leading to his capture was a sign of the importance that the United States placed on him.

Besides the new leader, the ISIS announcement also proclaimed a new spokesman, identified as Abu Hamza al-Qurayshi, again using a name indicating a lineage from the Prophet's tribe, and setting up another potential successor.

"Those names are the most generic names I can think of in a long time," Mr Raineri said. "This trick is obfuscating on purpose the possible links to people we know."

In the audio recording uploaded on the Telegram app, ISIS mourned the loss of Baghdadi, who led the organisation for nearly a decade, and its spokesman, Abu Hassan al-Muhajir, who was killed a day after Baghdadi and who had widely been considered a potential successor.

The audio recording was the first word from ISIS confirming the death of its leader, which President Donald Trump triumphantly announced on Sunday as a huge blow to the world's most fearsome terrorist group.

Mr Trump and Pentagon officials said Baghdadi had blown himself up with a suicide vest, also killing two children, after he had been cornered on Saturday in a dead-end tunnel during a US military raid in a northern Syrian village.

Al-Muhajir was killed on Sunday in an air strike elsewhere in northern Syria.

Baghdadi's death came eight months after US-led forces in Syria seized the last remnants of the territory once held by ISIS, which at its height spanned an area the size of Britain across parts of Syria and Iraq.

The announcement, in a 7min-37-sec recording, was coupled with a warning to the United States not to gloat over killing Baghdadi, who oversaw beheadings of American hostages and other atrocities.

"Do not be happy America, for the death of Sheikh al-Baghdadi, and do not forget the cups of death at his hands, may God accept him," the announcement said.

It boasted of the group's disciples and expansion beyond the Middle East, even as its core territory in Iraq and Syria was reduced to nil: "Don't you see America that the State is now on the threshold of Europe and Central Africa?"

Records of wire transfers and the testimony of captured fighters in the Democratic Republic of Congo indicate that the group has set up a base of operation in the Central African country.

The announcement also took aim at the leadership of Mr Trump, admonishing the United States: "Don't you see how you became the laughingstock of the nations, and an old and crazy man controls your fate, whose opinion changes between morning and evening?"