By David Brunnstrom, Simon Lewis and Alasdair Pal WASHINGTON (Reuters) - The United States, Australia, India and Japan recommitted to working together on Tuesday, after the first meeting of the China-focused "Quad" grouping's top diplomats since President Donald Trump returned to the White House.

On his first full day as U.S. secretary of State, Marco Rubio is meeting with his counterparts from Australia, India and Japan. The four countries are known as the Quad.

Rubio called China the "most potent, dangerous adversary" during his confirmation and is expected to work with India, Japan, and Australia to counter this.

China's relations are starting to improve with Japan, India and other countries that former U.S. President Joe Biden courted, just as Donald Trump brings his more unilateralist approach back to the White House.

Chinese President Xi Jinping reaffirmed ties with his Russian counterpart Vladimir Putin on Tuesday, as the two face a more geopolitically assertive US government led by Donald Trump, who criticised the Kremlin's war against Ukraine on the first day of his second term as president.

President Donald Trump widened his tariff threats to include China and the European Union on his second day back in office after day one saw Canada and Mexico in his sights.