WASHINGTON (Reuters) - The United States has asked Japan and NATO allies who have refused to send troops to Afghanistan to pay the estimated $17 billion needed to build up the Afghan army, according to U.S. defense officials.

Afghan National Army soldiers raise their rifles as they train in Kabul September 21, 2008. REUTERS/Ahmad Masood

The push to quickly increase the size of Afghanistan’s army and spread the cost of the initiative underscores the financial and military strain the war has placed on the United States and NATO members, many also operating in Iraq and elsewhere.

“The faster we get the (Afghan army) to the size and strength they need to be, the less they depend on us for providing security, and God knows we operate far more expensively than the Afghan national security forces do,” said Pentagon press secretary Geoff Morrell.

“At a minimum it’s going to cost $17 billion. That’s a hefty price-tag and someone’s got to pay it,” Morrell said.

“This may be one of those cases where countries that have had a reluctance to contribute forces, in particular combat forces, may be able to take part in this mission through a financial contribution to the development of the Afghan National Army.”

The new Pentagon push to share costs more widely reflects a realization among U.S. officials that some allies simply will not put troops into the war despite heavy pressure from Washington - something Europe has been telling the United States for more than a year.

But it also threatens to create just the type of two-tiered NATO alliance that U.S. Defense Secretary Robert Gates warned against early this year.

FIGHT, OR WRITE A CHECK

Gates in February said NATO risked a split between allies willing to “fight and die” and those who were not.

Morrell last week cast it as “those who fight and those who write checks.”

The NATO mission in Afghanistan will be discussed later this week at a meeting in Budapest. It will be the last NATO meeting of defense ministers during the Bush administration and Gates, who has repeatedly and publicly chided allies for two years to put more resources into the war, is expected to press the issue of cost sharing.

The meeting comes as the Bush administration conducts a comprehensive review of its strategy in Afghanistan, where security has steadily deteriorated despite a doubling of foreign troops over two years. Britain’s commander in Afghanistan, in fact, has said the war against the Taliban cannot be won.

U.S. officials said NATO was not conducting any parallel review, although the senior commander has just submitted an updated and secret list of troop and equipment requirements.

The United States has 33,000 troops in Afghanistan. About 22,000 are part of NATO’s force of nearly 48,000 troops. The United States contributes the most troops by far among allies, followed by Britain with about 8,000.

Commanders say they still need three more brigades, or about 10,000 to 12,000 troops. Those troops will most likely come from the United States next year, Morrell said.

The Afghan army plans to double in size to 134,000 soldiers over five years at a cost of $17 billion to $20 billion, according to estimates from U.S. officials.

The United States has already approached Japan about paying part of that bill. But Morrell said the request was made to Japan’s previous government, whose prime minister abruptly resigned in September, and that the request may have to be made again to the new government in Tokyo.

He did not say what other countries were asked to pay.