WASHINGTON - NATO's chief assured President Barack Obama on Tuesday of the alliance's commitment to the Afghan war as the U.S. administration weighs sending more troops to try to turn the tide on a resurgent Taliban.

Vowing NATO will stay in Afghanistan as long as it takes to finish our job, NATO Secretary-General Anders Fogh Rasmussen sought to ease Americans' doubts about whether their allies have the stomach to stay in the fight despite mounting military casualties.

But Rasmussen offered no pledge of additional forces.

Public opinion in both the United States and Europe has turned increasingly against what Obama's aides once hailed as the good war, compared with the highly unpopular war in Iraq that occupied the focus of his predecessor, George W. Bush.

Our operation in Afghanistan is not America's responsibility or burden alone: It is and it will remain a team effort, Rasmussen said after talks with Obama at the White House.

Obama, who will meet his Afghanistan war council on Wednesday, stressed the same point: This is not an American battle, this is a NATO mission as well.

Obama has said he will not decide on sending more U.S. troops to Afghanistan until after a broad review of his administration's approach.

The top U.S. and NATO commander in Afghanistan, General Stanley McChrystal, has offered a grim assessment, warning the eight-year-old war effort will likely fail without a significant change in strategy that would also involve a troop buildup.

McChrystal is believed to be seeking 30,000 to 40,000 combat troops and trainers, according to defense and congressional officials.

Rasmussen said he agreed with Obama's approach of strategy first, then resources.

But in a sign of misgivings about the war among U.S. allies, Rasmussen said in a speech on Monday that European countries were likely to be more comfortable contributing trainers than combat troops. He stressed, however, that they had added 9,000 troops in the last 18 months.

Since taking office in January, Obama has already almost doubled U.S. troops in Afghanistan to 62,000 to contend with the worst violence since U.S.-led forces ousted the Taliban rulers from power in 2001.

Obama now faces a fierce debate on the troops issue within his own administration as well as opposition from some of his fellow Democrats to an escalation of U.S. forces. A decision is expected to take at least several weeks.

WAR COUNCIL

Senior advisers and U.S. commanders were meeting on Tuesday to prepare for Wednesday's opening round of a series of sessions with Obama, who has promised tough questions.

White House spokesman Robert Gibbs said Obama would consider McChrystal's assessment plus additional ideas.

McChrystal will address the White House strategy session via video link. With Obama for the talks will be Vice President Joe Biden, Defense Secretary Robert Gates, Secretary of State Hillary Clinton, special envoy Richard Holbrooke and Joint Chiefs of Staff Chairman Mike Mullen, among others.

Biden has proposed a more narrowly focused strategy of using drone missile strikes against al Qaeda cells in the Afghanistan-Pakistan border areas while increasing training of Afghan forces.

In New York, Kai Eide, the U.N. special envoy to Afghanistan, told the Security Council more international troops were needed to train the Afghan army and police and that providing them cannot be a U.S. effort alone.

Underscoring the security threat, a roadside bomb killed 30 people in southern Afghanistan on Tuesday, officials said.

More than 1,500 civilians have been killed in Afghanistan so far this year, the United Nations said.

It said 68 percent of the civilian killings were a result of militant attacks, while 23 percent were caused by Afghan and foreign troops led by NATO and the U.S. military.

Attacks by insurgents have become deadlier this year than at any time in the eight-year-old war.

Polls show support among Americans for the war, launched by Bush after the September 11 attacks on the United States in 2001, has waned. A Gallup survey last week said 50 percent of Americans opposed sending more troops to Afghanistan, with 41 percent supporting it.

Obama's decision on whether to send more troops has been made more complicated by a dispute over the Afghan presidential election, and a final call over what strategy to follow is unlikely to be made until that dispute is resolved.

Afghan election officials have sent for ballot boxes to conduct a partial recount of the August 20 vote, after charges of widespread fraud and rigging.

Preliminary results gave President Hamid Karzai 54.6 percent of the vote, but a U.N.-backed fraud watchdog has ordered an audit of the results from 12 percent of polling stations, where suspiciously large numbers of votes were cast or one candidate got 95 percent.

If enough votes for Karzai are nullified so that he no longer has 50 percent, he would face a second-round runoff against his main challenger, former Foreign Minister Abdullah Abdullah.

(Additional reporting by Jeff Mason in Washington, Michelle Nichols in New York and Ismail Sameem, Golnar Motevalli and Sayed Salahuddin in Afghanistan; Editing by Simon Denyer and John O'Callaghan)