NATO Secretary General Jens Stoltenberg said on Thursday the alliance had made "no final decision" on the future of its Afghanistan mission, as new US President Joe Biden weighs pulling out troops. 

While defence ministers held off on making that call at a two-day virtual conference, they did decide to expand a NATO training mission in Iraq from 500 to "around 4,000" personnel. 

The fate of NATO's 9,600-strong support mission in Afghanistan was top of the agenda after former US leader Donald Trump struck a deal with the Taliban to pull troops out.

Biden's administration is reviewing whether to stick to a looming May 1 deadline to withdraw or risk a bloody backlash from the insurgents by staying.

"We have made no final decision on the future of our presence," Stoltenberg said at a news conference. 

"But as the May 1st deadline is approaching, NATO allies will continue to closely consult and coordinate in the coming weeks."

Stoltenberg insisted the Taliban must live up to commitments under the deal with the US, including making progress in peace talks with Kabul, reducing violence and cutting ties to "international terrorist groups".

"NATO's goal is to ensure that Afghanistan never again becomes a safe haven for terrorists that would attack our homelands," he said.

On Iraq, defence ministers agreed to bolster a NATO training mission aimed at strengthening the country's military as it looks to curb the resurgence of the Islamic State group. 

"The size of our mission will increase from 500 personnel to around 4,000," Stoltenberg said, adding that the increases would be "incremental". 

"Training activities will now include more Iraqi security institutions, and areas beyond Baghdad."

Stoltenberg insisted that the mission was being carried out "at the request of the Iraqi government". 

"It is carried out with full respect for Iraq's sovereignty and territorial integrity," he said.