UNITED NATIONS (Reuters) - The United Nations defended on Wednesday its recommendation that the 15-member U.N. Security Council postpone a planned visit to Afghanistan this month over security concerns after some diplomats questioned the U.N. threat assessment.

Diplomats said the decision to postpone the trip came after U.N. security chief Gregory Starr recommended earlier this month that they not travel to Afghanistan in the coming weeks, citing concerns for the safety of council diplomats.

Some council diplomats privately questioned the U.N. recommendation as vague, but others told Reuters it would be unwise to question Starr's assessment of the risk level in Afghanistan.

U.N. spokesman Farhan Haq said a security assessment had been made to the council based on the best information available.

"We stand by the recommendation, but will not comment further," he said.

U.S.-backed Afghan forces toppled the Taliban government in late 2001 when it refused to hand over al Qaeda militants, including Osama bin Laden, after the Islamist network's hijacked airliner attacks on the United States on September 11 that year.

The U.N. Security Council unanimously extended on Tuesday its authorization of a NATO-led International Security Assistance Force for another year. The United Nations also has a political, development and aid mission in Afghanistan.

Foreign troops have started handing over security control to Afghan soldiers and police, a process due to be completed by the end of 2014. A smaller NATO-led training and advisory mission is expected to remain, but NATO has given no details yet.

Russia warned on Wednesday it will stop cooperating with NATO on Afghanistan after 2014 unless the alliance gets U.N. Security Council authorization for its new training mission in Afghanistan.

A NATO official said only that it would be "helpful" to have a U.N. Security Council resolution authorizing the post-2014 training mission, but stopped short of saying it was essential.

(Reporting by Michelle Nichols and Louis Charbonneau; Editing by Eric Beech)