Days after the Islamic State’s 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 a new leader and warn America: “Do not be happy.”

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

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

Trump and Pentagon officials said al-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 an American military raid in a northern Syrian village.

Al-Mujahir was killed on Sunday in an airstrike elsewhere in northern Syria.

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

The Islamic State announcement said that al-Baghdadi had been succeeded as leader by Ibrahim al-Hashemi al-Qurayshi, whom it identified as the “emir of the believers” and “caliph”. Al-Qurayshi appeared to be largely unknown, at least outside the ranks of the organisation.

The al-Qurayshi appellation at the end of the new leader’s name indicates that he is being portrayed as a descendant of the Quraysh tribe, a lineage that the Islamic State considers to be a prerequisite for becoming a caliph or ruler.

Its use indicates that the IS continues to see itself as a caliphate, even if it is one with practically no territory.

The announcement was coupled with a warning to the US not to gloat over killing al-Baghdadi, who oversaw beheadings of American hostages and other atrocities.

“Do not be happy O America, for the death of Sheikh al-Baghdadi, and do not forget the cups of death at his hands, may God accept him,” said the announcement, according to a translation by the Site Intelligence Group, which monitors jihadist content online.

“Do you not realise, O America, that the Islamic State today stands at the threshold of Europe and Central Africa? It is expanding and remaining, with permission from God, from the east to the west.”