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 on Tuesday said Baghdad would not accept any security agreement with the US unless it included dates for the withdrawal of foreign forces.
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.
Perino said she would not put a timetable on when the security agreement 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," she said.
Nuri al-Maliki, the Iraqi prime minister, said on Monday that a military agreement the two countries are negotiating should include provisions for the withdrawal of American 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.