In his first major military move, President Barack Obama Tuesday approved the deployment of 17,000 more troops to Afghanistan, saying they were needed "to stabilize a deteriorating situation."

"There is no more solemn duty as President than the decision to deploy our armed forces into harm's way," Obama said in a statement.

"I do it today mindful that the situation in Afghanistan and Pakistan demands urgent attention and swift action.

"This increase is necessary to stabilize a deteriorating situation in Afghanistan, which has not received the strategic attention, direction and resources it urgently requires," he said.

Obama said the deployment orders were in response to a months-old request by the US commander in Afghanistan, General David McKiernan, who had asked for 30,000 more troops.

He said that "to meet urgent security needs" he had approved a request by US Defense Secretary Robert Gates to deploy a Marine Expeditionary Brigade in the spring and an Army Stryker Brigade and support forces later this summer.

"The Taliban is resurgent in Afghanistan, and Al Qaeda supports the insurgency and threatens America from its safe-haven along the Pakistani border," he said.

The White House said some 17,000 troops will deploy to Afghanistan ahead of the Afghan national elections scheduled for August 20, significantly building up the 38,000 US force battling the spreading insurgency.

They include 8,000 marines in the marine expeditionary brigade, 4,000 troops in the army Stryker brigade, and 5,000 support troops.

Obama indicated that the units being sent to Afghanistan had been earmarked for Iraq, saying the drawdown of US forces there "allows us the flexibility to increase our presence in Afghanistan."

The deployment decision comes amid a comprehensive review of US strategy in Afghanistan and Pakistan, but Obama said it would not pre-determine the outcome of the review.

"Instead, it will further enable our team to put together a comprehensive strategy that will employ all elements of our national power to fulfill achievable goals in Afghanistan," he said.

"As we develop our new strategic goals, we will do so in concert with our friends and allies as together we seek the resources necessary to succeed," he said.

It was unclear how the additional troops will be used, with many warnings that there is no military solution to Afghanistan's growing problems.

Senator John McCain, Obama's rival for the presidency, welcomed the move but expressed hope that it was "just the first step in a new comprehensive approach to Afghanistan."

"A major change in course is long overdue," he said.

The US-backed government in Kabul has come under an intensifying challenge as the insurgency has gained strength and spread from the east and south into parts of the west and areas around the capital.

US intelligence has warned that endemic corruption and the government's inability to deliver services and protect the populace has eroded its legitimacy.