BEIJING — In a sharp rebuff, China accused Washington on Wednesday of taking Japan’s side in a tense clash over disputed islands in the East China Sea, underscoring rising regional friction as visiting Vice President Joe Biden met with Beijing’s leaders.

Emerging from a private meeting with President Xi Jinping that went considerably longer than scheduled, Biden appeared somber and subdued. In a brief appearance before reporters in which he took no questions, Biden did not go into details on differences over China’s newly declared restricted flying zone. Instead, he spoke of a “new model of major country cooperation,” saying U.S.-China relations must hinge on trust and a positive notion of each other’s motives.