The United States President Barack Obama has vowed to send 30,000 more troops to Afghanistan at the "fastest pace possible" to target Taliban insurgency and secure key cities.
But Mr Obama has warned the US will begin to withdraw its military from the eight-year war by 2011, the BBC reports.
The new deployment, beginning early next year, will bring America's troop strength in the country to more than 100,000.
Mr Obama reached his decision after more than three months of deliberations and 10 top-level meetings with advisers.
General Stanley McChrystal, the US commander in Afghanistan, welcomed the speech, saying he had been given "a clear military mission" and the necessary resources.
Not made lightly - Obama
Making the announcement at the West Point Military Academy in New York State on Tuesday night, Mr Obama said he did not make the decision lightly.
He said he had written to parents of soldiers killed in Iraq and Afghanistan, read parents' letters to him, visited wounded soldiers and met soldiers' coffins arriving back in the United States.
"If I did not think that the security of the United States and the safety of the American people were at stake in Afghanistan, I would gladly order every single one of our troops home tomorrow."
The president also increased pressure on NATO allies for more troops and cautioned that Afghans must ultimately take responsibility for their conflict-ravaged country.
Pentagon officials hope other NATO members will send an extra 10,000 troops.
Mr Obama says the US troops will also train Afghan forces, so they can take full responsibility for the country's security.
Allies will boost numbers - Nato head
The head of NATO Anders Fogh Rasmussen says he expects allies to provide at least 5000 troops for Afghanistan and possibly a few thousand more.
Mr Rasmussen welcomed President Obama's announcement of a new US approach and commitment to the Afghan mission.
He says there are 43 countries there under NATO command and he's confident other allies and partners will make a substantial increase in their contributions.
The British prime minister Gordon Brown has said Britain will deploy another 500 troops this month.
New Zealand has 220 soldiers in Afghanistan, of which 82 are SAS specialists.
'Risky move' for president
An American foreign policy analyst says Mr Obama's decision to send more troops could determine his future in the White House.
Bruce Riedel, of the think-tank Council of Foreign Relations, told Radio New Zealand's Morning Report programme wars always consume presidencies.
Mr Riedel, who also chaired a White House review of policy toward Afghanistan and Pakistan, believes the decision to send more troops is risky for Mr Obama.
"This is a big gamble by the president - there's no assurance of success. But the alternatives - cutting and running, or staying where he is - are even worse.
"This White House has embarked on a gamble which really puts the presidency at stake."