NATO head Jens Stoltenberg said on Wednesday that the Manchester bombing shows the alliance must agree, at a summit with US President Donald Trump, to do more to combat terrorism.
Terrorism will be top of the agenda at Thursday's meeting in Brussels which comes amid sharp divisions over joining the US-led coalition against Daesh.
"I expect NATO allies to step up and agree to do more in the fight against terrorism, not least because of the attack we saw in Manchester," Stoltenberg said at a news conference on the eve of the summit.
Trump arrived in Brussels on Wednesday having said NATO was "obsolete" because it did not focus on the threat of terrorism.
He has since softened his criticism but still wants NATO to join the anti-Daesh coalition itself as an important gesture of support for the campaigns in Syria and Iraq.
All 28 allies have joined the coalition as individual countries and if NATO became a member, that would significantly boost coordination in the war against Daesh in Syria and Iraq, Stoltenberg recalled.
He said the "brutal attack" claimed by Daesh showed the terror threat remained ever-present.
"Many allies would like to see NATO as a full member of the coalition," Stoltenberg added.
"Firstly, because it sends a strong message of unity ... and especially in light of the attack in Manchester, I think it is important to send this message of unity against terrorism," he said.
NATO currently provides AWACS surveillance planes to help anti-Daesh operations and trains officers in Iraq but it stresses that these are and should remain non-combat roles.
Diplomatic sources say some of the allies, including France and Germany, are reluctant to go further for fear of getting dragged into a ground war and risking NATO's standing with Arab powers.
They are also concerned NATO could end up taking over control of the whole operation in Iraq.
New French President Emmanuel Macron is due to meet Trump for lunch on Thursday when the issue is expected to be a major talking point.
May to attend summit
British Prime Minister Theresa May will attend the G7 and NATO summits as planned despite the Manchester attack, Downing Street said on Tuesday.
On Friday and Saturday, Group of Seven leaders from the United States, Japan, Britain, France, Germany, Italy and Canada will meet in Sicily.
"G7 and NATO summits continue as planned," May's spokesman told reporters.
Asked if there would be any change to the issues discussed, the spokesman said: "Counter-terrorism is already a very significant part of the agenda."
The UK prime minister was already expected to lead a discussion on counterterrorism at the G7.