Iraqi government forces have reportedly been forced to pull back from the northern city of Tikrit after an offensive to reclaim the city from Sunni Muslim militants was blunted by fierce fighting.

The BBC, citing eyewitnesses, reported Sunday that the Iraqi army had fallen back to the town of Dijla, approximately 15 miles to the south. The witnesses said that the government forces' drive to retake Tikrit had been hampered in part my the large number of improvised explosive devices laid on the approach to the city. The witnesses said that security forces continued to shell the city, though what damage those shells may have done is unclear.

Iraqi officials had claimed that troops had pushed into the heart of the city in what was the biggest attempt yet by the Baghdad government to reverse a series of defeats suffered earlier this month at the hands of the Al Qaeda-inspired Islamic State in Iraq and The Levant (ISIS). The militants have surged across Sunni-majority areas of the north and west, menacing the capital and bringing the government of Prime Minister Nouri al-Maliki to the brink of collapse in the process.

Tikrit, the hometown of former Iraq dictator Saddam Hussein, was overrun by ISIS fighters on June 11 as part of an offensive that also swallowed up Iraq's second-largest city, Mosul.

Saturday's fighting began before dawn with helicopter gunships carrying out airstrikes on insurgents who were attacking troops at a university campus on Tikrit's northern outskirts, Iraqi military spokesman Lt. Gen. Qassim al-Moussawi said. The government forces had established a bridgehead on the university's sprawling grounds after being airlifted in the previous day.

Sporadic clashes continued throughout the day at the university. At the same time, several columns of troops pushed north toward Tikrit from Samarra, a city along the banks of the Tigris River and home to an important Shiite shrine, a senior security official said on condition of anonymity because he was not authorized to brief the media.

By sundown, Lt. Gen. Ahmed Abu Ragheef, a commander in the Salahuddin Operational Command, said a column of troops had reached the edge of Tikrit, while another had secured an air base that previously served as a U.S. military facility known as Camp Speicher.

The governor of Salahuddin province, Ahmed Abdullah al-Jabouri, told The Associated Press that troops pushed into Tikrit itself, reaching the provincial council building.

However, residents reached by telephone Saturday evening said militants were still in control of Tikrit, a predominantly Sunni city of more than 200,000, and patrolling the city's streets.

They confirmed the clashes around the university, and reported fighting between the Islamic State and Iraqi forces to the southeast of the city as well. Some residents described black smoke rising from a presidential palace complex located along the edge of the Tigris River after army helicopters opened fire on the compound.

They spoke on condition of anonymity out of concern for their safety.

Many locals had already fled the city in anticipation of a government assault, said another Tikrit resident, Muhanad Saif al-Din.

"Tikrit has become a ghost town because a lot of people left over the past 72 hours, fearing random aerial bombardment and possible clashes as the army advances toward the city," Saif al-Din said. "The few people who remain are afraid of possible revenge acts by Shiite militiamen who are accompanying the army. We are peaceful civilians and we do not want to be victims of this struggle."

He said the city has been without power or water since Friday night.

The military also carried out three airstrikes on the insurgent-held city of Mosul early Saturday. Mosul is Iraq's second-largest city, and was the initial target of the Islamic State's offensive in the country.

The airstrikes come after a senior U.S. Defense official confirmed to Fox News that the United States is flying armed drones over Iraq, though the aircraft are not being sent at this time to engage Sunni militant fighters.

The U.S. already was flying surveillance drones over the country, and the decision to dispatch armed drones marks an escalation. But the armed drones, at this stage, still are only in place for surveillance purposes.

The Iraq government received an added boost in its fight against the militants with the delivery Saturday in Baghdad of five Russian-made Sukhoi jets. Two Iraqi security officials confirmed arrival of the planes, which Iraq purchased secondhand from Russia.

"We are in urgent need of this type of aircraft during this difficult time," said Iraqi air force commander Lt. Gen. Anwar Hama Amin. "These jets will enter service within a few days -- the coming three or four days -- in order to support the units and to fight the terrorist ISIL organization."

Video provided by the Iraqi Defense Ministry late Saturday showed the jets being unloaded from a large cargo plane.

South of Baghdad, heavy clashes between security forces and Sunni insurgents in the town of Jurf al-Sakhar killed at least 21 troops and dozens of militants, police and hospital officials said on condition of anonymity because they were authorized to brief the media. Jurf al-Sakhar, located some 30 miles outside the capital, is part of a predominantly Sunni ribbon that runs just south of Baghdad.

The Islamic State, which already has seized control of vast swaths in northern and eastern Syria, aims to create a state straddling Syria and Iraq governed by Islamic law. In Iraq, the group has formed an alliance of sorts with fellow Islamic militants as well as former members of Saddam's Baath party to fight al-Maliki's Shiite-led government.

The militants have tapped into deep-seated discontent among Iraq's Sunni community with al-Maliki, who has been widely accused of monopolizing power and alienating Sunnis.

The United States and other world powers have pressed al-Maliki to reach out to the country's Sunni and Kurdish minorities and have called for a more inclusive government that can address longstanding grievances.

Al-Maliki is fighting to retain his post, which he has held since 2006, as many former allies drop their support and Iraqis increasingly express doubts about his ability to unify the country. But he appears set on a third consecutive term as prime minister after his bloc won the most seats in April elections.

Fox News' Jennifer Griffin and The Associated Press contributed to this report.