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Washington (CNN) -- President Barack Obama is expected to announce this week that 30,000 U.S. "surge" forces will be fully withdrawn from Afghanistan by the end of 2012, an administration official told CNN.

Members of Congress are being informed that the 10,000 troops will be withdrawn by the end of this year, followed by another 20,000 next year, a congressional source said.

Obama will deliver a highly anticipated speech on the troop drawdown at 8 p.m. ET Wednesday.

The time frame would give U.S. commanders another two "fighting" seasons with the bulk of U.S. forces still available for combat operations. Outgoing Defense Secretary Robert Gates has pushed for additional time to roll back Taliban gains in the country before starting any significant withdrawal -- a position at odds with a majority of Americans, according to recent public opinion surveys.

Gates -- along with Afghan war commander Gen. David Petraeus -- had pushed for an initial drawdown of between 3,000 and 5,000 troops this year, the congressional source said. The secretary also urged the president to withdraw support troops only -- not combat troops.

Obama, however, ultimately decided to adopt a more aggressive withdrawal plan.

Gates acknowledged Tuesday that the president must take into account public opinion and congressional support for further military engagement.

"Sustainability here at home" is an important consideration, Gates said, noting that people are "tired of a decade of war."

Sen. Carl Levin, D-Michigan, chairman of the Senate Armed Services Committee, repeated his call Tuesday for a withdrawal of 15,000 troops this year.

"The level of U.S. troop reductions in Afghanistan needs to be significant to achieve its purpose --- letting the Afghan government know we are determined to shift primary responsibility for their security to the Afghan security forces," Levin said in a written statement.

An estimated 100,000 U.S. troops are serving in Afghanistan, some 30,000 of which are part of the so-called surge ordered in 2009 in a bid to control the rising violence.

Obama has been mulling how many troops should be withdrawn this summer and by the end of the year.

The president is expected, in his remarks Wednesday, to stress the importance of preserving flexibility in force levels on the ground so commanders can adjust as conditions warrant, the official said.

The drawdown will be accomplished by troops returning home and not being replaced as well as canceling some proposed deployments.

So far, Obama has only said publicly that troops will begin coming home in July, and he recently indicated the number would be "significant."

The president has repeatedly said he is confident the United States can meet the self-imposed deadline to begin bringing troops back from Afghanistan without compromising Afghan security, though military commanders and government officials have raised concern about the readiness of Afghan security forces.

"We have made great strides toward achieving the objectives laid out in the mission that the president articulated in December of 2009," White House press secretary Jay Carney said Monday.

Obama "will make his decision based on the need to succeed further in achieving those objectives and to transfer authority gradually, security authority, over to the Afghan national security forces, with an eye to the fact that, as agreed to by NATO in Lisbon, we will eventually transfer full security lead over to the ANSF (Afghan National Security Forces) in 2014," Carney said.

Nearly three-quarters of Americans polled this month said they support the United States pulling some or all of its forces from Afghanistan.

That figure jumped 10 percentage points since May, likely as a result of the death of Osama bin Laden, pollsters said.

Republicans -- who have been the strongest supporters of the U.S. military presence in Afghanistan -- appear to be shifting their opinion on the conflict. Forty-seven percent of Republicans said in a CNN/Opinion Research Corp. poll in May that they favored a partial or full withdrawal of American troops. Sixty percent of Republicans favored a withdrawal when asked this month.

The most recent CNN/Opinion Research Corp. poll was conducted June 3 through 7, with 1,015 adult Americans questioned by telephone. The survey's overall sampling error is plus or minus 3 percentage points.

CNN's Barbara Starr, Ted Barrett and Alan Silverleib contributed to this report.