The US general commanding NATO forces in Afghanistan warned Thursday that he needs thousands more troops and accused Russia of seeking to prop up the Taliban.

US-led forces have been fighting in Afghanistan for 16 years, making it already America's longest-ever war, but General John Nicholson told Congress: "I believe we're in a stalemate."

The question of whether to double down in the seemingly endless conflict will now become one of the first major military strategy questions of President Donald Trump's term.

Trump has not laid out any detailed new strategy for Afghanistan in his first weeks in office, but during the campaign, he reluctantly accepted that US troops would remain.

And last month, the Wall Street Journal reported that Trump had suggested in a call to Afghanistan's President Ashraf Ghani that he would consider sending more soldiers. The pair spoke again on Thursday.

"The leaders discussed opportunities to strengthen the bilateral relationship in areas such as security, counterterrorism cooperation, and economic development," the White House said.

Trump's spokesman Sean Spicer told reporters that the president would seek the advice of Defense Secretary James Mattis before deciding on Nicholson's request for reinforcements.

"I think the president will heed the advice of the generals and Secretary Mattis. That conversation has yet to happen," he said.

Testifying before the US Senate Armed Services Committee, Nicholson said he has a "shortfall of a few thousand" troops needed to train, advise and assist Afghan government forces.

The NATO alliance has 13,300 troops in Afghanistan, about half of them American, assisting a much larger Afghan force in a war against the Taliban and other Islamist militants.

- Afghan casualties -

Afghan government police and armed forces now take the lead in the battle against the Taliban, which was forced from power in Kabul after the 2001 US-led invasion.

US battlefield deaths are relatively rare since the transfer of primary responsibility to Afghan forces.

But the Afghans are facing mounting casualties -- their losses in 2016 were up by a third over 2015, with 6,785 killed in the first 10 months of the year -- and losing ground.

Last year was also particularly deadly for Afghan civilians. A UN report found that 11,500 were killed or injured in 2016, the most since it began keeping records in 2009.

Nicholson told the committee that having more US and allied troops would allow him to train and advise Afghan units down to a brigade level and in ministries in Kabul.

The general said he was discussing the request for reinforcements with his own chain of command and that Mattis would talk to the NATO allies.

- 'False narrative' -

The Senate panel, led by hawkish committee chairman John McCain, were also keen to hear more about the alleged malign influence of Russia and Iran in the Afghan conflict.

Trump has repeatedly said he hopes to mend ties with Russia's President Vladimir Putin so that Russia and the United States can work together against "radical Islamic terror."

But according to Nicholson, Russia is instead giving the Taliban encouragement and diplomatic cover in order to undermine American influence and to defeat NATO.

The general told the senators that Russia was trying to "legitimize" the Taliban by promoting a "false narrative" that the rebels are fighting the Islamic State group.

Reports say Russian officials held several meetings with Taliban representatives in Tajikistan and Moscow in recent months.

Russia also hosted talks on Afghanistan with officials from Pakistan and China in Moscow without inviting Ghani's government, harming efforts to build an Afghan-led peace process.

"When we look at Russia and Iranian actions in Afghanistan, I believe that in part they're to undermine the United States and NATO," Nicholson said.

- 'Not the right time' -

In Moscow, the Kremlin's special envoy for Afghanistan, Zamir Kabulov, said it was "not the right time to withdraw foreign soldiers from Afghanistan, including American soldiers."

"The security situation in the country is worsening, and the Afghan forces are unable to resist the armed opposition by themselves," he told the state news agency TASS.

Kabulov said there were around 15,000 "extremists" in northern Afghanistan, and warned they could "try to expand into territory in neighboring countries in Central Asia."

He reiterated that Russia was willing to cooperate with the United States and NATO members in Afghanistan.

Russia, which fought its own nine-year war in Afghanistan in the 1980s as the Soviet Union, announced on Tuesday that it plans to host a conference of regional powers.

Foreign Minister Sergei Lavrov said Moscow would invite representatives from Afghanistan, Pakistan, China, Iran and India and insisted the Taliban must be included in peace talks.