WASHINGTON (CNN) -- US President Barack Obama is sending 30,000 additional troops to Afghanistan and is ordering military officials to get the reinforcements there within six months, White House officials told CNN Tuesday.

The president, whom Republicans had accused of "dithering" over the decision, came to the conclusion that the deployment needs to be accelerated to knock back the Taliban, the officials said.

The push for a speedy deployment surprised some observers, because White House officials who defended Obama's slow pace of coming to a decision had said the Pentagon wouldn't be able to get new troops to Afghanistan that quickly anyway.

Asked to explain that seeming contradiction, a White House official told CNN: "The president is saying this has to happen, so the military will make it happen."

The president will travel to the U.S. Military Academy in West Point, New York, Tuesday night for a speech to the nation on his strategy for Afghanistan.

The officials said the president will give the American people some sort of "time frame" for getting out of Afghanistan. White House aides had said earlier this week there would not be a timetable in the speech.

"He will talk about specific dates" to withdraw from the war, according to the officials.

As for why the president chose West Point as the venue, the officials noted the Army has borne an extremely heavy burden in this war, so this is an important symbol.

The officials said West Point not only is where these cadets train, but also is where they study counterinsurgency principles.

The officials contended the president "brought divergent voices together" in the debate over Afghanistan strategy.

The decision to send 30,000 additional service members could delay the Army's promise of ensuring all troops get at least two years home between deployments, a senior Army official told CNN.

The Army's goal was to implement such a policy by 2011, the official noted.

In the speech, Obama is expected to announce he'll seek further troop commitments from NATO allies as part of a counterinsurgency strategy to wipe out al Qaeda elements and stabilize the country, while training Afghan forces.

The expected new troop deployment would increase the total U.S. commitment in Afghanistan to almost 100,000 troops, bolstered by about 45,000 NATO forces.

According to White House press secretary Robert Gibbs, the president's speech will explain why the United States is involved in Afghanistan, the new American mission in the war-torn country, and the process that led to Obama's decision.

Obama also will emphasize the limit on U.S. resources in manpower and budget, and stress that the Afghan mission is not open-ended, Gibbs said.

The decision carries significant political risk for Obama, who will announce it nine days before he travels to Oslo, Norway, to accept the Nobel Peace Prize.

His liberal base, which helped him win last year's presidential election, opposes another troop deployment to Afghanistan.

"I think he's made up his mind that there needs to be a troop increase, and I have to say I'm very skeptical about that as a solution," said Rep. Janice Schakowsky, D-Illinois, a longtime Obama ally who now worries Afghanistan will become what she calls another quagmire.

In addition, the deployment -- expected to cost $30 billion a year -- comes amid high unemployment as the economy emerges from a recession. That concerns Democrats and Republicans faced with competing domestic priorities such as health care reform and job creation.

House Appropriations Committee Chairman David Obey, D-Wisconsin, recently proposed a special war surtax to finance the conflict.

Gibbs told reporters Monday that he had "not heard extensive discussion" at the White House about a possible surtax.

"I know the president will touch on costs" during Tuesday's address, he said, but "I don't expect to get overly detailed [about that issue] in the speech."

In Afghanistan, reaction to the possibility of more U.S. troops ranges from outright opposition to a willingness to see what happens.

"We welcome their arrival if they really expel the Taliban, terrorists, and al Qaeda from the borders of Afghanistan," said Mohammad Zia, 40, in Kabul, the capital. "But if they come and kill more civilians and destroy villages, then they shouldn't come."

Back home, Obama's allies said the president must convince the American public that sending more troops will help achieve the goals of the mission.

"The president needs to explain how more combat troops will speed up training of Afghan forces," Sen. Carl Levin, D-Michigan, said Sunday on the CBS program "Face the Nation."

The deployment won't work if the mission is for the United States to take on the Taliban on its own, Levin said. Gibbs said Obama has been briefing top aides, military officials and foreign leaders about his decision.

Obama previously ordered more than 20,000 additional troops to Afghanistan.

U.S.-led troops first invaded Afghanistan in response to the September 11, 2001, attacks on New York and the Pentagon by the al Qaeda terrorist network. The invasion overthrew the ruling Taliban, which had allowed al Qaeda to operate from its territory -- but most of the top al Qaeda and Taliban leadership escaped the onslaught.

Taliban fighters have since regrouped in the mountainous region along Afghanistan's border with Pakistan, battling U.S. and Afghan government forces on one side and Pakistani troops on the other. Al Qaeda's top leaders, Osama bin Laden and Ayman al-Zawahiri, remain at large and are suspected to be hiding in the same region.

The conflict has claimed the lives of more than 900 Americans and nearly 600 allied troops.

CNN's Ed Henry, Barbara Starr, Atia Abawi, Alan Silverleib and Tom Cohen contributed to this report.