(For more on Afghanistan, click [ID:nAFPAK])
* Obama to consider extra troops when strategy defined
* McChrystal warns without more troops mission could fail
* Karzai, United Nations welcome McChrystal's assessment (Recasts with range of options on the table, previous WASHINGTON/KABUL)
By Adam Entous
WASHINGTON, Sept 22 (Reuters) - The White House is weighing a range of options to stem Taliban gains in Afghanistan, from sharply increasing U.S. ground forces to stepping up aerial attacks on targets in Pakistan, or a combination of the two, officials said on Tuesday.
The top-to-bottom review of U.S. options has taken some military officials by surprise, and is holding up transmission of a request by the top U.S. and NATO commander in Afghanistan, Army General Stanley McChrystal, for additional troops.
"The president first wants to fully discuss his assessment of the situation in Afghanistan and the strategy we are pursuing there before considering any additional resources for that effort," said Pentagon press secretary Geoff Morrell.
McChrystal, the top commander of U.S. and NATO forces in Afghanistan, warned in a confidential assessment leaked to the media on Monday that without additional troops the mission would "likely result in failure."
But Obama, who committed himself in March to a broader counterinsurgency strategy, described himself in weekend interviews as a "skeptical audience" when it came to the issue of deciding whether to send more troops.
There are already more than 100,000 foreign soldiers in Afghanistan battling an insurgency that has taken control of parts of the south and east of the country in what has so far been the deadliest year for foreign troops since the war began in 2001.
The leaking of McChrystal's military report, which Afghan President Hamid Karzai has said he backs, piles more pressure on Obama, already squeezed by ebbing public support and skepticism in his own party over troop levels.
Some administration and congressional officials said they expected McChrystal's request to include roughly 30,000 new combat troops and trainers.
Administration officials, speaking on condition of anonymity, said McChrystal's troop recommendations would be seriously considered, along with other options. A major change in strategy would require a different mix of resources.
Obama's civilian and military advisers increasingly see the war against al Qaeda and the Taliban as a "single battlefield" that runs from Afghanistan into the tribal areas of Pakistan and requires a more unified approach.
Under Obama, the CIA has already sharply increased U.S. attacks on al Qaeda and Taliban targets in Pakistan using unmanned aerial drones.
Military officials said any expansion would have to be done cautiously because it could inflame rampant anti-American sentiment in Pakistan and undermine U.S. efforts.
U.S. commanders have also been pressing for a more concerted push against militants on both sides of the border.
The United States has praised Pakistan's "success" in recent operations against militants in the Swat valley, northwest of Islamabad.
But Pakistan has put on hold a push into the South Waziristan region on the Afghan border, the main base for Pakistani Taliban fighters loyal to Baitullah Mehsud, who was killed in a CIA missile strike last month.
Islamabad has cited shortages of military equipment, including Cobra attack helicopters and precision weapons, and U.S. officials say they are trying to free up the equipment that Pakistan needs.
McChrystal's assessment comes at a critical time for Afghanistan, when the war is at its deadliest since 2001 and as Afghans await delayed presidential election results now being audited by a U.N. watchdog following fraud accusations against incumbent Hamid Karzai.
In an interview with news channel CNN, Karzai supported McChrystal's assessment and said the commander's call for more troops was "the right approach ... and we back it".
McChrystal's report places fresh emphasis on protecting Afghan civilians and engaging their support.
Some 800 Afghan civilians were killed between January and May this year alone, the United Nations says, with just over a third of these casualties caused by international and Afghan forces.
"We welcome what McChrystal has indicated, that protecting Afghan civilians forms the centerpiece of military strategy," said Aleem Siddique, a spokesman for the United Nations in Kabul. (Additional reporting by Golnar Motevalli in Kabul, Caren Bohan in Washington and Luke Baker in London; Editing by Simon Denyer and Jackie Frank) (For more Reuters coverage of Afghanistan and Pakistan, see: here)