The Iraqi army has launched an assault on the Sunni rebel-held town of Tikrit, a day after releasing video footage of soldiers in control of a strategic northern oil refinery.

Exclusive video obtained on Friday by Al Jazeera shows damage inside Tikrit after reports of Iraqi military helicopters flying commandos into the town on Thursday. A fixed-wing aircraft was also recorded flying over the town.

Reuters reported that fierce clashes began after the helicopers landed near Tikrit's university campus. At least one of the helicopters was downed by the rebels, according to the news agency.

Ahmed al-Jubbour, a professor at the university's college of agriculture, described to Al Jazeera fighting in the colleges of agriculture and sports education after three Iraqi army helicopters landed.

"I saw one of the helicopters land opposite the university and I saw clashes between dozens of militants and government forces," he said.

However, several locals told Al Jazeera that there were no rebels in the region and that the military hit targets indiscriminately.

"They dropped barrel bombs here, we were home and all of a sudden two blasts took place. There is no one here. No militants here in the region," one resident told Al Jazeera.

Human Rights Watch meanwhile says nearly 190 of captured Iraqi army recruits were executed by fighters when the town fell to ISIL on June 10 and 11.

The rights group analysed satellite imagery and photos released by the fighters to reach the figure. Social media accounts sympathetic to the group claimed up to 1,700 had been executed.

"The number of victims may well be much higher, but the difficulty of locating bodies and accessing the area has prevented a full investigation," Human Rights Watch said.

Refinery takeover

The Iraqi army released a video on Thursday showing it in control of the Baiji oil refinery, 200km north of Baghdad.

Major-General Qassim Atta al-Moussawi said the army repelled attackers on Wednesday and "killed all the militants who approached the perimeter of Baiji refinery."

"Baiji refinery has turned into a graveyard for the coward militants," Moussawi said.

Baiji lies in territory captured about two weeks ago by the ISIL fighters and its Sunni allies. ISIL still controls the town of Baiji, which is next to the refinery.

Meanwhile, the president of the Kurdish region of Iraq, Masoud Barzani, said on Friday there was no going back on Kurdish rule of Kirkuk, a city in an oil rich area of Iraq which Kurdish Peshmerga forces took control of earlier this month.

Grand Ayatollah Ali al-Sistani, the top Shia cleric in Iraq, also stated on Friday that Iraq needed to chose a prime minister to lead a new government, due to be sworn in after negotiations starting on on July 1.

Nouri al-Maliki, the Iraqi prime minister, has refused to step down, saying any attempt to undermine him would be considered a coup.