The military operations in Iraq and Afghanistan could cost $170 billion in the next fiscal year over and above the $515.4 billion regular Pentagon budget that President Bush has proposed, Defense Secretary Robert M. Gates said on Wednesday.

Mr. Gates gave that estimate in testimony before the Senate Armed Services Committee after cautioning the panel that any estimate would be dicey, given the unpredictability of war.

“Well, a straight-line projection, Mr. Chairman, of our current expenditures would probably put the full-year cost in a strictly arithmetic approach at about $170 billion,” Mr. Gates said in response to questions from Senator Carl Levin, the Michigan Democrat who is the head of the committee.

So, Mr. Levin pressed, “That would be a total then of $685 billion” in Pentagon spending for the fiscal year that begins Oct. 1. “Does that sound right?”

“Yes, sir,” Mr. Gates replied. “But as I indicated, I have no confidence in that figure.”

Mr. Levin has been a persistent critic of the war in Iraq, and he has complained that the Bush administration has been less than straightforward about the financial costs of the Iraq and Afghanistan campaigns by seeking supplemental funding outside the regular Pentagon budget. Congress has gone along with the supplemental requests, with members of both parties pledging to give American troops whatever they need.

“While the monetary cost is not the most important part of the debate over Iraq or Afghanistan, it does need to be part of that debate, and the citizens of our nation have a right to know what those costs are projected to be,” Senator Levin said.

Mr. Gates said he was concerned that some countries who have pledged troops to Afghanistan were not fully meeting their commitments, and that he would bring up the subject with his counterparts from other NATO countries.

“I think we have to be realistic about the political realities that face some of the governments in Europe,” Mr. Gates said. “Many of them are coalition governments, some of them are minority governments, and they are doing what they think is at the far end of what is politically acceptable.”