WASHINGTON (AP) — When it comes to fighting the Islamic State militants, the Obama administration likes to claim strength in numbers — specifically the 66 countries in its coalition.

But a year and a half into the war, some administration officials are acknowledging that this supposed source of strength has its own weaknesses.

Defense Secretary Ash Carter offered a glimpse of his own apparent frustration last week when he referred to "our so-called coalition."

He says more coalition members need to do more.

With that shortcoming in mind, Carter is traveling to Brussels to convene a meeting Thursday of defense chiefs from about two dozen coalition countries, including most NATO members and Persian Gulf states, Iraq among them. He will urge them to step up their contributions.