He declined to say when operations for all US troops would be completed, saying it depended on when the Iraqi government completes certain tasks, such as purchasing its own aircraft.
"I would not put an X on the calendar, Mr Chairman," Dubik said when asked to name a date when all US forces would complete operations in Iraq.
Dubik, who is retiring, said that Iraqi security forces had grown to 566,000 personnel as of May 2008, up from 444,000 in June 2007.
The Iraqi military said on Wednesday that the number of "terrorist attacks" in June was down 85 per cent from the same period a year ago.
Security gains in Iraq "are dramatic, but can be reversed and can be stymied", Dubik said.
"Some form of partnership and assistance ... in my opinion is still necessary."
He cautioned that the fast-growing Iraqi force lacked experienced leaders and the ability to train its own troops.
Dubik's observations came on a day that Dana Perino, White House spokeswoman, told reporters travelling with George Bush, the US president, in Japan that the US remains opposed to "an arbitrary [troop] withdrawal date".
The US believes those decisions should be "based on conditions on the ground" and Iraqi officials agree with that, she said.
Iraq's national security adviser said on Tuesday that Baghdad would not accept any security agreement with the US unless it included dates for the withdrawal of foreign forces.
|Dubik said that Iraq's forces lacked experienced leadership [EPA]
But the government's spokesman said any timetable would depend on security conditions on the ground.
Their differences underscored the debate in Baghdad over the security pact with Washington that will provide a legal basis for US troops to remain when a UN mandate expires at the end of the year.
The White House said the statements from Iraqi officials about a timetable for troop withdrawal partly reflected improvements in the security situation in Iraq.
"I think that is a reflection of first and foremost the positive developments that we've seen recently in Iraq, but in addition to that, the negotiations are intensifying," Perino said.
"This is about their future and they want to take on more of their own responsibility, and we want that too," she said.
She said she would not put a timetable on when the security pact might be completed.
"We want to be able to try to work this out quickly and the main reason that we want this is because our troops are going to be there past the end of this year, that's a fact," Perino said.
The US position appears to be at odds with that of Nuri al-Maliki, the Iraqi prime minister, who said on Monday that the pact should include provisions for the withdrawal of US troops.
In a meeting with Arab ambassadors in the United Arab Emirates, al-Maliki said Iraq had proposed a short-term interim memorandum of agreement rather than the more formal status of forces agreement the two sides have been negotiating.
"Negotiations are ongoing with the US side and the current attitude is to reach a memorandum of understanding either for immediate US forces withdrawal or timetable withdrawal," he said.
The memorandum "now on the table" includes a formula for the withdrawal of US troops, he said.
"The goal is to end the presence [of foreign troops]," al-Maliki said.