The European Union leader was seen stumbling, walking awkwardly and losing his balance several times ahead of a gala dinner Wednesday.
Juncker was stumbling and swaying to the extent that the presidents of Finland and Ukraine, among others, had to help keep the 63-year-old upright. Dutch Prime Minister Mark Rutte also stepped in to guide Juncker.
Juncker has said he suffers from sciatica that sometimes affects his movements. Portuguese Prime Minister Antonio Costa said Thursday that Juncker had complained of back pain.
"He doesn't have a serious health problem as far as I am aware of but he does have a back problem for some time," Rutte said. "It is a problem in a sense that he sometimes has these pain attacks."
Juncker's office declined to comment, citing privacy concerns.
Leaders of European institutions were invited to join NATO leaders for a meeting and dinner Wednesday night.
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