WASHINGTON -  The United States won't attend a multinational peace conference on Afghanistan next month in Russia, a State Department official said.

The reasons: The US wasn't consulted before receiving the invitation and doesn't know Russia's objectives for the gathering.

The official told Associated Press that Washington wants to work with Moscow on regional efforts to end the 16-year war, and that Secretary of State Rex Tillerson would bring up the matter when he visits Russia in April. The official wasn't authorised to speak publicly on the matter and demanded anonymity.

Afghanistan, Pakistan, Iran, India and several Central Asian nations are among the invitees to the Moscow conference. Afghan and US officials say the Taliban aren't invited. The State Department hasn't publicly announced its position on the planned conference.

Last year, Pakistan, Afghanistan, China and the United States met to jump-start the peace process but that effort faltered.

The official said the US wants nations in the region, which have a shared interest in peace in Afghanistan, to increase pressure on the Taliban to negotiate with the Afghan government.

Last week, Pakistani officials hosted seven Taliban leaders in Islamabad to try to press the insurgents into peace talks ahead of the Moscow meeting, two Taliban officials told the AP.

Islamabad has been under international pressure to try to bring Taliban leaders, who have lived in Pakistan since their rule in Afghanistan was overthrown in the 2001 US invasion, into some form of negotiations with Kabul.

In Washington on Tuesday, Afghan Foreign Minister Salahuddin Rabbani called for the US to add to the 8,400 troops currently in Afghanistan — where the Taliban have stepped up attacks and the Islamic State group also poses a threat.

He said the Afghan government remains open to peace talks but doubted the Taliban would participate unless Pakistan cracked down on "terrorist safe havens" on its soil — a long-running source of bitterness between the neighboring countries.

Meanwhile, Russia on Friday denied allegations by the commander of NATO that Moscow may be assisting the Taliban as the insurgents fight US and NATO forces in Afghanistan.

"These claims are absolutely false," Zamir Kabulov, head of the Russian foreign ministry's department responsible for Afghanistan and the Kremlin's special envoy in the country, told RIA Novosti state news agency in Moscow.

"These fabrications are designed, as we have repeatedly underlined, to justify the failure of the US military and politicians in the Afghan campaign. There is no other explanation."

NATO's Supreme Allied Commander, US General Curtis Scaparrotti, who also heads the US military's European Command, told lawmakers in Washington on Thursday that he had witnessed Russia's influence grow in many regions, including in Afghanistan.

In a statement to the Senate Armed Services Committee, Scaparrotti said Moscow was "perhaps" supplying the Taliban.

In February General John Nicholson, the US commander of NATO forces in Afghanistan, testified that Russia is encouraging the Taliban and providing them with diplomatic cover in a bid to undermine US influence and defeat NATO.

Kabulov in 2015 said that Russia was exchanging information with the Taliban and saw shared interest with them when it comes to fighting the Islamic State jihadist group.

Russia considers the Taliban a terrorist group and it is banned in the country, along with the Islamic State group.

Taliban fighters on Thursday captured Afghanistan's strategic district of Sangin, where US and British forces suffered heavy casualties until it was handed over to Afghan personnel.