Militants who have seized a large swathe of northern and north-central Iraq now plan to march on the capital Baghdad, a US-based monitoring group said Wednesday.

In a lightning offensive, Islamic State of Iraq and the Levant militants seized Tikrit earlier, its latest success following a spectacular assault late Monday on Mosul, a city of two million.

The militants' advances have forced as many as half a million people to flee their homes.

ISIL spokesman Abu Mohammed al-Adnani promised that the battle would "rage" on Baghdad and Karbala, a city southwest of the capital that is considered one of the holiest sites for Shiite Muslims, the SITE Intelligence Group said.

"Do not relent against your enemy... The battle is not yet raging, but it will rage in Baghdad and Karbala," Adnani said, according to a SITE translation of an audio statement released on the militants' Twitter feed.

"Put on your belts and get ready."

Adnani also dismissed Prime Minister Nuri al-Maliki as woefully incompetent, calling him an "underwear salesman."

"What have you done to your people, O foolish one. No one is more foolish than you but those who accept you as the president and commander," Adnani said in the translated statement.

"What do you know about policy, leadership, and military command? You lost a historic opportunity for your people to control Iraq, and the Shiites will always curse you for as long as they live. Indeed, there is between us and you a balance to be made even."

Iraqi officials have already privately asked the US to consider sending in drones to root out ISIL militants, American officials said.

Washington has vowed to boost aid to Iraq and is mulling drone strikes amid fears Iraqi forces are crumbling in the face of militants increasingly emboldened since the US military withdrew in late 2011.

Tikrit -- hometown of executed dictator Saddam Hussein -- was the second provincial capital to fall in as many days as the militants and their allies captured a string of mainly Sunni Arab towns where resentment against the Shiite-led government runs deep.

After Tikrit's fall, the operation spread down the main highway towards Baghdad, with militants battling security forces on the northern outskirts of Samarra, just 70 miles (110 kilometers) from the capital.

State television said security forces responded with air strikes, and residents said the fighting subsided without the militants entering the city.

Samarra is mainly Sunni Arab but is home to a shrine revered by the country's Shiite majority, whose bombing by Al-Qaeda in 2006 sparked a Shiite-Sunni sectarian conflict that left tens of thousands dead.