BAGHDAD - Iraqi forces launched an offensive on Tuesday to drive Islamic State fighters out of Tikrit, hometown of executed dictator Saddam Hussein, while the militants warned they would attack Americans "in any place".
In Geneva, the United Nations refugee agency said it was starting a major aid operation to get supplies to more than half a million people displaced by fighting in northern Iraq.
Buoyed by an operation to recapture a strategic dam from the jihadists after two months of setbacks, Iraqi army units backed by Shi'ite militias fought their way towards the centre of Tikrit, a city 130 km (80 miles) north of Baghdad which is a stronghold of the Sunni Muslim minority.
"Our forces are advancing from two directions with a cover from army helicopters, mortar and artillery shelling the positions of the Islamic State fighters in and around the city," an army major in the operations room told Reuters.
Sunni Muslim fighters led by the Islamic State swept through much of northern and western Iraq in June, capturing the Sunni cities of Tikrit and Mosul as well as the Mosul dam, a fragile structure which controls water and power supplies to millions of people down the Tigris river valley.
However, on Monday fighters from Iraq's Kurdish autonomous region said they had regained control of the hydro electric dam with the help of US airstrikes.
The Islamic State has concentrated on taking territory for its self-proclaimed caliphate in Syria and Iraq unlike al Qaeda - the movement from which it split - that has repeatedly attacked US targets including New York and Washington on Sept. 11, 2001.
However, in a video posted on the Internet warned Americans in English that "we will drown all of you in blood" if US air strikes hit its fighters. The video also showed a photograph of an American who was beheaded during the US occupation of Iraq that followed Saddam's overthrow in 2003 and victims of snipers.
The Iraqi major said fierce fighting was underway near Tikrit's main hospital 4 km (2.5 miles) from the city centre of the city. "Helicopters are pounding the bases of the terrorists to prevent them from regrouping," he said.
As well as a push from the south, Iraqi forces were advancing only slowly from the west due to land mines and roadside bombs planted by the militants, he added. A police captain confirmed the details of the fighting.
MAJOR AID PUSH
The UNHCR refugee agency said a four-day airlift of tents and other goods would begin on Wednesday to Arbil, capital of the Kurdish autonomous region, from the Jordanian port of Aqaba. This would be followed by road convoys from Turkey and Jordan and sea shipments from Dubai via Iran over the next 10 days, UNHCR spokesman Adrian Edwards said.
"This is a very, very significant aid push and certainly one of the largest I can recall in quite a while," he told a news briefing in Geneva. "This is a major humanitarian crisis and disaster. It continues to affect many people."
Iraqi government forces put up little serious resistance when the Islamic State staged their June offensive, while Kurdish fighters also suffered setbacks until President Barack Obama ordered the US air strikes earlier this month.
Obama said he acted to protect Americans and prevent a genocide in a conflict that has forced hundreds of thousands of Iraqis to flee their homes, including from the Yazidi and Christian religious minorities.
Baghdad is now trying to turn the tide after the Kurds said they had taken the dam, easing fears that the militants could cut off electricity and water supplies, or even breach the structure, causing huge loss of life and damage down the Tigris.
Efforts are underway in Baghdad to form a new government that will unite the majority Shi'ites with the Sunnis and Kurds in halting the Islamic State insurgency that threatened to tear the country apart.
Sunnis long dominated Iraq until the US-led invasion forced Saddam to flee. He was captured near Tikrit in late 2003 and executed in 2006.