"The point is we're at a deadline, and we need an answer," Navy Adm. Mike Mullen told reporters traveling with him in Iraq.
Under a security agreement between Iraq and the United States, all U.S. troops are to be out of the country by Dec. 31.
Iraqi leaders recognize security forces aren't completely prepared to defend the country from insurgents or other nations, Mullen said, so they need to make a decision about asking for help from U.S. forces beyond the withdrawal date.
"It's pretty clear that we've said to the Iraqi leadership that now's the time ... we have to know" if they're going to request a continuing U.S. presence, he said.
Roughly 48,000 U.S. personnel are training, advising and assisting Iraqi army and police units, the Pentagon said in a release. But the logistics of removing U.S. troops and equipment before the Dec. 31 deadline mean Iraqi leaders must decide soon whether to ask that some U.S. troops remain past the deadline, Mullen said.
U.S. officials have said they don't expect any formal request until September.
An Iraqi request would start negotiations and any U.S. decision would have to consider the security environment, what capabilities the Iraqis need and what the legal status of American forces would be, as well as the ability to protect U.S. service members, Mullen said.
Mullen's tour includes stops at Mosul and Baghdad.