Baghdad, Iraq (CNN) -- A man purported to be the notorious leader of the so-called Islamic State appeared at a mosque in Mosul, calling on people to carry out a holy war, according to witnesses and a video posted to the extremist group's website.
A video promoted on social media sites tied to the Islamic State in Iraq and Syria purports to show Abu Bakr al-Baghdadi leading Friday prayers at the Great Mosque of al-Nuri in Mosul, a move that comes weeks after ISIS seized control of Iraq's second-largest city.
If it is al-Baghdadi, it would be one of the first known appearances of the militant leader to be captured on video. Very few images have been made public of al-Baghdadi.
The Iraqi government has seen the video and is analyzing it, a spokesman in Prime Minister Nuri al-Maliki's office told CNN. The U.S. government also is looking into it.
In the video, the man purported to be al-Baghdadi urges people to follow his call to create a "caliphate," an Islamic state, in Iraq and Syria, and wage "jihad" during Ramadan -- the holy month of fasting for Muslims that began last week.
"God created us to worship him and spread his religion, and ordered us to fight his enemies for him and for religion," the man said.
During the sermon, the man said Muslims are sinners if they did not seek this goal of establishing an Islamic state.
"I am being entrusted with this heavy burden and duty to be your imam, although I am not the best one among you," he said.
"Therefore, if you see me as righteous, you should help me and if you see me wrong, you should advise me and help me. Obey me as long as long as I obey God in ruling you. ...If I disobey God, then you are no longer obligated to obey me."
Al-Baghdadi's appearance comes days after ISIS declared him as the leader of its new "caliphate," or Islamic state, extending from Aleppo in northeastern Syria to the Diyala province in Iraq.
The extremist group also said it was changing its name from the Islamic State of Iraq and Syria to "the Islamic State."
CNN cannot independently verify the authenticity of the video, which shows dozens of people, including men who appear to be bodyguards, listening to the sermon and praying. Several automatic weapons are displayed near the man delivering the sermon.
Al-Baghdadi was never identified by name, only as "the emir of the believers," according to a woman who was at the mosque when he arrived. CNN is identifying the witness only as Bara'a out of a concern for her safety.
"It was a terrifying Friday," she said.
The video identifies the man as Al-Khalifah Ibrahim, the name al-Baghdadi now goes by with his followers since ISIS declared the creation of the so-called "Islamic State" -- a move that enraged many Muslims.
Four to five minutes before the sermon was to begin, the imam at the mosque was told, in front of followers, to step down, Bara'a told CNN.
Then, black vehicles with blacked-out windows pulled up out front of the mosque, she said. The men who got out of the vehicles were chanting "Allahu Akbar" (God is great) as they walked in.
"The emir of the believers, he will give the sermon," said one of the men who arrived with al-Baghdadi. "So listen to him."
Those who accompanied al-Baghdadi took over the first row in the mosque, she said. In the video, the faces of the people in that row are blurred.
Bara'a was sitting on the second floor of the mosque, an area designated for women to gather during the sermon.
"We were crying quietly, fearing they will figure out we were crying out of fear and that they would harm us," she said.
"We were kept in the mosque for an hour. There were a number of gunmen who blocked the door."
Very little is known about al-Baghdadi, but a biography posted on jihadist websites last year said he earned a doctorate in Islamic studies from a university in Baghdad.
He formed the militant group in Salaheddin and Diyala provinces north of the Iraqi capital before joining al Qaeda in Iraq.
Al-Baghdadi was detained for four years in Camp Bucca, which was a U.S.-run prison in southern Iraq. He was released in 2009.
CNN's Tim Lister, Arwa Damon, Raja Razek, Ali Younes and Gul Tuysuz contributed to this report.