WASHINGTON - The United States will not be in Afghanistan eight years from now, the White House said on Wednesday, as President Barack Obama prepared to explain to Americans next week why he is expanding the war effort.

After months of deliberation and fending off Republican charges that he was dithering on Afghanistan while violence there surged, Obama will address the nation on Tuesday on the way forward in the costly and unpopular eight-year war.

He is expected to announce he is sending about 30,000 more troops as part of a new counterinsurgency strategy that will place greater emphasis on accelerating the training of Afghan security forces so that U.S. soldiers can eventually withdraw.

It appears highly unlikely Obama will offer a specific troop withdrawal timetable, but White House spokesman Robert Gibbs said the president would stress that the U.S. involvement in Afghanistan was not open-ended.

We are in year nine of our efforts in Afghanistan. We are not going to be there another eight or nine years, Gibbs told reporters. Our time there will be limited and that is important for people to understand, he said.

He said Obama would use his prime-time televised speech to stress the sheer cost of the war, explain to Americans why their military was still in Afghanistan, and press Afghan President Hamid Karzai to improve governance after being re-elected in a fraud-tainted vote in August.

The American people are going to want to know why we are here, they are going to want to know what our interests are, Gibbs said.

The White House has estimated it will cost $1 million per year for each additional soldier sent to Afghanistan. With the U.S. deficit hitting $1.4 trillion and fueling Americans' concerns about high government spending, sending more troops to Afghanistan could be a politically risky move for Obama.

Obama's fellow Democrats, who control the U.S. Congress, face potentially difficult midterm elections in November 2010, with Republicans eager to exploit Americans' unease about the country's ballooning deficit and high unemployment.

Two veteran Democratic lawmakers have already called for imposing a war tax to pay for the troop increase.

VERY, VERY, VERY EXPENSIVE

Gibbs said Obama would meet with key lawmakers to brief them about his plan ahead of his Tuesday speech. Key committees in the House of Representatives and the Senate will hold back-to-back hearings next Wednesday and Thursday with Secretary of State Hillary Clinton, Defense Secretary Robert Gates and Joint Chiefs of Staff Chairman Admiral Mike Mullen.

Gibbs said the financial cost of the conflict -- which reached $6.7 billion in June alone -- and the physical toll it had taken on the U.S. military made the war unsustainable in the long term.

It is very, very, very expensive, Gibbs said.

Obama will again press Karzai to improve the performance of his corruption-plagued government. Karzai's legitimacy was tarnished after a fraud-riddled election in August that saw millions of ballots favoring him thrown out.

As the president has told President Karzai, there has to be a new chapter in Afghan governance and that is something the president will talk about on Tuesday, Gibbs said.

Obama has spent the past three months reviewing the U.S. strategy in Afghanistan, where a resurgent Taliban has driven violence to its highest levels since U.S. forces invaded in 2001 to oust the militant Islamists for harboring al Qaeda leaders responsible for the September 11 attacks on the United States.

The president has drawn fire from Republican critics for the time he has taken to reach a decision, but the White House has countered saying the former Bush administration neglected Afghanistan and allowed the security situation to deteriorate.

Obama's address to the nation at 8 p.m. EST on December 1 (0100 GMT December 2) from the West Point military academy in New York state will mark the end of a long process of deliberation that was characterized by a slow drip of leaks about the various options he was considering.

Angered by the leaks, which some analysts saw as an attempt by some in the administration to influence the president's thinking, Obama threatened to make them a firing offense.

(Additional reporting by Adam Entous; editing by Patricia Wilson and Eric Beech)

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