Any operation to take Mosul will be "Iraqi-led and U.S.-supported," he said.
"It's important that it be launched at a time when it can succeed and so I think the important thing is that it get done when it can be done successfully. Even if I knew exactly when that was going to be, I wouldn't tell you," he said.
"Of course, I'm open. I'm always open to advice from our military commanders about what the best way to achieve success is.
"And that is a question that will come down the road but I think what's important is that the campaign to retake Mosul succeed and we're committed to that success and not to a particular timetable."
Months after most Iraqi troops dropped their weapons and ran for their lives in Mosul, up to 25,000 are expected to head back there in April or May to try to retake the city, according to a U.S. official.
And Iraqi Prime Minister Haider al-Abadi told the BBC
this week that while there's still work to do, he is sure the Iraqis are going to retake the crucial northern city.
"We are now planning an offensive on Mosul in the coming few months," the Prime Minister said. "We have to prepare for it carefully because the only choice we have in Mosul (is to win). We have to win in Mosul to keep (ISIS) out."
U.S. Secretary of State John Kerry, speaking in London, said the international community was increasingly determined to act together against ISIS and that progress would continue to be made.
"We have the tools, we have the political will, we have the determination and we are making gains in Iraq. Territory is increasingly beginning to come back into the hands of the Iraqi government. The Iraqi military is now beginning to stand up with greater capacity," he said.
ISIS militants seize rifles, armored vehicles
Despite Kerry's vote of confidence, some doubt whether victory in Mosul is possible.
Mosul has been a symbol of Iraqi military incompetence, given how troops and police ran from their posts as ISIS militants arrived
Since then, there has been the start of airstrikes by the United States and international allies against ISIS.
The Iraqi military has also had fresh training, some of it conducted by U.S. and allied forces, to make it more effective. It has had some success in curbing the ISIS onslaught, but not in taking back a lot of territory -- much less that as valued as Mosul.
New footage released by ISIS on Friday gave another insight into the challenge facing the Iraqi army and its allies.
The video, apparently of an attack early this month on an Iraqi military outpost near Samarra, 100 miles north of Baghdad, showed dozens of seized American-made M16 assault rifles and piles of AK-47s, as well as armored personnel carriers and a Humvee.
All these are now in the hands of ISIS militants who appear to have overrun the outpost, some 60 miles northwest of Baghdad. The video also showed several dead Iraqi soldiers, with one of the bodies burning.
U.S. Central Command (CENTCOM) said it had no confirmation of when and where the vehicles were seized or where the video was actually shot.
Although this latest incident was only small, ISIS forces have steadily acquired large quantities of U.S. weaponry as a result of seizing Iraqi army equipment provided by the United States, particularly in Mosul.
Is Iraqi army ready for the fight?
Up until now, one of the biggest and most successful forces has been the Kurdish Peshmerga that serve Iraq's semiautonomous Kurdish government, often at odds with Baghdad.
On Saturday, Kurdish fighters repelled a massive offensive by ISIS in the town of Gwer and gained full control of the area, the Kurdistan Democratic Party (KDP) Foreign Relations Office told CNN. At least 34 ISIS militants were killed during the fighting.
These Kurdish fighters will have a role, including around Mosul. In his BBC interview, al-Abadi said that liaisons between Iraqi forces and the Peshmerga "must be powerful and must be watertight" to defeat ISIS.
According to the CENTCOM official, who spoke to reporters Thursday, Mosul police and tribal forces would likely join Iraqi troops in the assault on the city. Peshmerga would play a supporting role, not going into Mosul but instead blocking off supply and escape routes north and west of the city.
The hope is for an operation in April or May to avoid running into Ramadan (mid-June through mid-July) and Iraq's summer heat, the CENTCOM official said. A final decision has not yet been made, though.
Count Sirwan Barzani, a senior Peshmerga commander, is among the skeptics that such a spring assault would work.
"I don't think it's realistic, and I don't have any idea about a plan," Barzani told CNN's Ben Wedeman. "And if it involves the Iraqi army only, it's not going to work. The Iraqi army is not ready for the fight."
Retired Maj. Gen. James "Spider" Marks told CNN he believed it had been a mistake to reveal the potential timing of the Mosul operation and that it could put U.S. forces at risk.
"This is a terribly unfortunate lack of professionalism, in almost a relaxed approach towards a challenge that we have been trying to get our arms around for almost a year," he said.
U.S. official: Up to 2,000 ISIS in Mosul
If the Iraqi forces -- from five army brigades -- do the street-to-street fighting, theoretically they should significantly outnumber their ISIS counterparts.
Right now, ISIS has an estimated 1,000 to 2,000 fighters in Mosul, the U.S. official said, but more could join the fight if they take the threat of attack seriously.
However, an additional challenge for the Shia-dominated Iraqi military and Peshmerga is that they will be trying to take a majority Arab Sunni population city.
Even if the offensive succeeds, sectarian divisions could exacerbate an already complicated situation and make it hard for the Iraqi military to hold on to the city.
The approximate time of the attack had previously been reported: A CENTCOM official told CNN earlier this year that Iraqi forces could make their first move in April.