The US military insisted Friday it was not pressurizing Iraq to launch an offensive to recapture Mosul from Islamic State extremists in the next few months, saying the timing was up to Baghdad.

Previous comments by senior US officials suggesting the Iraqi army would stage a counter-attack against the IS group in the northern city as soon as April or May have provoked an angry response in Baghdad.

But Pentagon press secretary Rear Admiral John Kirby said Washington was not out to force the hand of the Iraqi army before it was ready to launch such a major undertaking.

"I don't think it's fair to say that the Pentagon or the military has been pushing the Iraqis on any specific timeline," Kirby told reporters.

"We haven't laid a date certain down here at the Pentagon... we're not pushing or aggressively trying to nudge them towards a faster timeline than they're going to be ready."

General Lloyd Austin, who as head of US Central Command oversees the international campaign against the IS jihadists, recently said that Iraqi and Kurdish forces would be ready by the spring or early summer to start an offensive to grab back Mosul -- Iraq's second-largest city, seized by the IS group last June.

And an official with Central Command told reporters last week that "the mark on the wall we are still shooting for is the April-May timeframe."

The idea was to stage the operation before the Muslim fasting month of Ramadan, which starts in mid-June, and before the summer heat kicks in, the official said.

A few days later, Iraq's defence minister, Khaled Obeidi, expressed irritation that the Pentagon would publicly announce a date for the battle of Mosul.

"A military official should not disclose the date and time of an attack," Obeidi said.

Analysts and former military officers said the Pentagon might be trying to sow fear among the IS group and to build an aura of inevitability about a future operation.

The former supreme allied commander of NATO, James Stavridis, told CNN that Washington may be seeking to "build a narrative" by forecasting the timing of an operation. But he called the move "a mistake."

The controversy coincided with the arrival of a new defense secretary in Washington, Ashton Carter, who conferred with top commanders and diplomats on the war effort on Monday in Kuwait.

When asked, Carter said he would not publicly discuss the possible timing of any offensive to recapture Mosul.

Iraq truck bomb attack kills 11: army
Samarra, Iraq (AFP) Feb 28, 2015 - A suicide truck bomb killed at least 11 Iraqi forces south of Tikrit Saturday during operations paving the way for an assault on the jihadist-held city, the army said.

"Three suicide bombers driving tanker trucks sped towards a gathering of security forces at a checkpoint," an army lieutenant colonel told AFP on condition of anonymity.

He said the attack took place near Sur Shnas, an archaeological site on the banks of the Tigris between the cities of Samarra and Tikrit, the main cities in Salaheddin province north of Baghdad.

"Security forces opened fire, managed to stop two but the third got closer to the group and blew himself up," the officer said.

Clashes ensued, he said, but "security forces successfully completed preparations to surround Ad-Dawr", a town just south of Tikrit which is controlled by the Islamic State jihadist group.

A hospital source in Samarra confirmed the toll of 11 dead.

The officer said a large operation would soon begin by army forces and volunteer fighters -- known as the Popular Mobilisation units -- aimed at retaking Tikrit, one of the jihadists' main hubs in the country.

Government forces have attempted and failed several times to wrest back Tikrit -- the hometown of former president Saddam Hussein -- since losing it to IS in June last year.

"We call on the people of Tikrit to leave their city within 48 hours to wrap up the battle of the revenge for Speicher," said Hadi al-Ameri, commander of the Popular Mobilisation.

His organisation confirmed the truck attack and said it caused casualties but did not say how many.

Speicher is a military base near Tikrit from which hundreds of new, mostly Shiite, recruits were kidnapped before being executed in the early days of the IS nationwide offensive in June 2014.

Shiite militias in particular have always vowed to avenge the Speicher executions, sparking fears of mass killings against Sunnis if Tikrit were to be recaptured.

Ameri heads the Badr organisation, arguably the most powerful Shiite militia in Iraq, and wields direct influence over the interior minister, who is one of his lieutenants.

Five people were also killed further south in Balad Ruz, a town northeast of Baghdad in Diyala province, that Badr helped recapture in January.

Local council member Said Majid said five people were killed and 15 wounded when a car bomb and an improvised explosive device went off in a market area.

IS fighters no longer hold fixed positions in Diyala but can still pose a security threat with such bomb attacks, as they do in the capital Baghdad.