Prime Minister Shinzo Abe and U.S. President Barack Obama talked on the phone Thursday morning about the last week's unofficial U.S.-China summit in California, a government official said.

In their 30-minute chat, Obama said he discussed U.S.-Chinese and Japanese-Chinese relations as well as North Korean issues with Chinese President Xi Jinping at the unofficial summit, the official said.

The ongoing tension between Japan and China over the Senkaku islets in the East China Sea and North Korea's nuclear arms and missile threats were raised during the summit, the official said. China and Taiwan also claim the Japan-controlled islets in the East China Sea.

Abe and Obama agreed to cooperate closely on North Korean issues and also agreed it is important for Japan and China to engage in dialogue, the official explained.

The official said Abe told Obama that Japan's door for dialogue with China is always open.

The source, however, declined to elaborate, saying Japan is not in a position to disclose the contents of the U.S.-China summit.

As for recent media reports that Xi termed the Senkakus "a core interest" of China's in the summit, the official said he wasn't aware of any such declaration.

Tokyo's position is that no territorial dispute exists over the islets, which Japan took control of in 1895, and Washington has said its security pact with Japan covers the islets.