The head of NATO is invoking this week's attack in Manchester, England, in a call for the alliance to focus more on fighting terrorism ahead of President Trump's first meeting with the alliance on Thursday.

"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," NATO Secretary-General Jens Stoltenberg said at a news conference previewing the meeting.

On Monday night, a suicide bomber detonated explosives as concertgoers were leaving an Ariana Grande performance, killing 22 and injuring 59.

The following day, the United Kingdom raised its threat level to "critical" — its highest level — signaling the possibility of an “imminent” attack. And on Wednesday, British officials said they believe the bomber was likely not acting alone.

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Trump had called NATO "obsolete" on the campaign trail, in part because he believes it has not done enough to fight terrorism. He backed off that criticism last month, saying the alliance is "no longer obsolete."

But with the Manchester attack coming just days before Trump's first NATO heads of state meeting, the alliance's efforts on terrorism are back in the spotlight.

One step NATO is expected to take is officially joining the U.S.-led coalition fighting the Islamic State in Iraq and Syria. All alliance members are members of the coalition, and NATO already provides training to Iraqi forces and surveillance planes known as AWACS. But the alliance is not a member of the coalition as its own organization.

On Wednesday, Stoltenberg said the “barbaric” Manchester attack puts into focus how joining the coalition can help, even symbolically.

“Especially in light of the attack in Manchester, I think it is important to send this message of unity against terrorism," he said.

Still, Stoltenberg said the discussion on whether to join the coalition is ongoing.

A few allies, including France and Germany, are reportedly concerned that the alliance joining the coalition will drag them into a ground war.

Regardless, Stoltenberg said he expects agreement on expanding AWACS support for the coalition.

“I expect we will agree to expand our AWACS support for the coalition,” he said. “This will contribute to airspace management, making the skies safer. I also welcome that several allies have committed air-to-air refueling capabilities for NATO AWACS supporting the coalition.”