U.S. oil prices plunged to their lowest level in more than a year after President Trump emphasized strong ties with the Saudi Arabian government.

West Texas Intermediate for January delivery fell 6.6% on Tuesday to $53.43 a barrel on the New York Mercantile Exchange, its lowest settle price since October 2017. Brent, the international benchmark, slid 6.4% to $62.53.

“After the United States, Saudi Arabia is the largest oil-producing nation in the world,” Mr. Trump said. “They have worked closely with us and have been very responsive to my requests to keeping oil prices at reasonable levels—so important for the world.”

Oil extended declines following Mr. Trump’s remarks and related comments about the U.S. remaining a “steadfast partner” of Saudi Arabia despite the killing of a dissident journalist in October.

Some market participants said Mr. Trump’s comments could be an attempt to keep Saudi Arabia from cutting production at the December meeting among OPEC and non-OPEC members. While the comments alone didn’t catalyze Tuesday’s selloff, they compounded existing concerns about oversupply, they said.