Islamic State militants have released what appears to be an audio recording of their leader, Abu Bakr al-Baghdadi.
A speaker who sounds like the IS leader seems to refer to recent North Korean threats against Japan and the US.
He also talks of battles for IS strongholds like Mosul, which was regained by Iraqi forces in July.
Baghdadi, who has a $US25 million US bounty on his head, has not been seen in public since July 2014, leading to much speculation about his fate.
The last time he appeared was to preach at the Great Mosque of al-Nuri in Mosul after IS overran the city and a "caliphate" was proclaimed.
Asked about the audio, a spokesman for US forces fighting IS, Ryan Dillon, said "without verifiable evidence of his death, we have continued to assume that he is alive".
A defence department spokesman said: "We are aware of the audio tape purported to be of Abu Bakr Al-Baghdadi and are taking steps to examine it. While we have no reason to doubt its authenticity, we do not have verification at this point."
The militant Sunni Muslim group IS, which earned notoriety for its brutal violence towards civilians and prisoners, has been steadily pushed back in Iraq and Syria this year.
The 46-minute recording was posted by a website with links to the group and is the first such speech to appear since November.
As well as Mosul, the speaker refers to battles in Raqqa and Hama in Syria, and Sirte in Libya, saying the bloodshed would not be in vain. He also talks of Russian-brokered peace talks on Syria. Much of the tape consists of religious references.
Less than a handful people would know his whereabouts, Hassan Hassan of the Tahrir Institute for Middle East Policy wrote in a recent article for BBC News.
That makes it hard for the US, which has dedicated special forces constantly on the look-out for him.
In June, Russia reported that it was "highly likely" Baghdadi had been killed in a Russian air force strike on Raqqa on 28 May, and an Iranian official asserted he was "definitely dead" shortly afterwards.
However, claims of his death had been made before and the reports were treated with scepticism by US officials.