NATO this week announced it wanted to send around 3,000 extra troops to Afghanistan, bringing the Western military footprint up to about 16,000 soldiers to help the Afghans break the stalemate in the 16-year fight against the Taliban.
But after talks with the alliance's 29 defence ministers in Brussels, Secretary General Jens Stoltenberg acknowledged there were still "gaps" in the roster that needed to be filled.
"We are now in the process of increasing the troop level," Stoltenberg said.
"We haven't finalised that so it's not possible to provide any final figures when it comes to how much different countries are going to contribute."
The additional troops, most of them American, will help train and advise local Afghan forces who have struggled to hold Taliban and ISIS extremists at bay while suffering heavy casualties.
Washington has asked NATO allies to contribute 1,000 extra troops for Afghanistan to add to 2,800 US forces, but diplomatic sources say they may have to be content with just 700.
Stoltenberg said NATO had made progress but was still in the process of gathering troop commitments.
"We got some new announcements from some nations during the meeting today but we still have some gaps that we will continue to work on," he said.
NATO leaders are optimistic that 2018 could see Afghan forces start to gain momentum against the Taliban, thanks to renewed training efforts, a growing air force and thousands of extra Afghan commandos.
Plus US President Donald Trump has given American forces greater leeway in how and when they can hit the Taliban, and Afghan forces are increasingly going on the offensive.
But the Taliban poured scorn on NATO claims of success.
"The Taliban are those who, in defence of their belief and country, forced the half-million NATO-led world forces equipped with modern weaponry and technology to kneel down," Taliban spokesman Zabihullah Mujahid said.
Mattis said coalition partners are looking to the United States for a clear plan about what follows the physical defeat of ISIS.
"Maybe three-quarters of the questions I am getting asked now is (about) going forward. It's not about are we going to be able to stop ISIS, are we going to be able to overcome ISIS. They are now saying: 'What's next? How is it looking?'" Mattis told reporters this week using another acronym for the group.
Following back-to-back losses, including of their Syrian and Iraqi strongholds of Raqa and Mosul, ISIS fighters are down to defending their last holdouts along the Euphrates River valley.
America's military involvement in Syria has until now been focused solely on fighting ISIS, but with the jihadists on the ropes, Washington must articulate its longer-term interests and what role, if any, US forces will play in Syria.
A French source said allies were keen to hear what Mattis had to say about the role of Iran -- a key supporter of Syrian President Bashar al-Assad -- following Trump's tough rhetoric against Tehran.
"We are wondering how the speeches by top US officials on the need to push back the Iranian presence in the region is going to manifest itself in real terms in the military strategy," the source said.
NATO members agreed Wednesday to increase the use of cyber weaponry and tactics during military operations, with the alliance also upgrading other capabilities to combat a resurgent Russia.
The changes are part of NATO's biggest shakeup since the Cold War, with defence ministers backing the creation of two new command centres to help protect Europe.
The North Korean nuclear crisis has also been high on the agenda in Brussels.
Tensions have soared since Pyongyang carried out its sixth nuclear test -- its most powerful to date -- and Stoltenberg said the crisis demanded a united international effort.