Patrick Shanahan took the top spot January 1 after Jim Mattis resigned amid disagreements with Trump.

Patrick Shanahan, the acting US defense secretary, started his first full day at the Pentagon's helm Wednesday by telling colleagues he sees China as a key priority.

Shanahan, who took the top spot January 1 after Jim Mattis resigned amid disagreements with President Donald Trump, told key Defense Department staff to focus on the National Defense Strategy, a broad review that highlights a new era of "Great Power competition" with Russia and China.

"While we are focused on ongoing operations, Acting Secretary Shanahan told the team to remember China, China, China," a defense official said.

The US accuses Beijing of an ongoing pattern of military and economic espionage, and has criticized China's ambitious "Belt and Road" trade and infrastructure initiative as being a form of economic coercion.

"In 2019, the National Defense Strategy remains our guide. America's military strength remains our focus," Shanahan said in a New Year's message on Twitter.

The defense official said Shanahan would later Wednesday attend a cabinet meeting with Trump, the president's first of 2019.

Little known outside business and Washington circles, Shanahan takes the world stage at a time of tumultuous changes and unpredictable foreign policy moves under Trump.

Shanahan did not serve in the military but has worked since July 2017 as the deputy defense secretary. Before that, he spent more than 30 years at Boeing.

Among the most immediate issues on his plate will be the pace at which the US pulls 2,200 or so troops out of Syria, following Trump's decision last month to exit the war-torn country.

According to US officials, Trump is also mulling a 50 percent draw down in Afghanistan -- another momentous move that has left lawmakers and international allies fearing for what comes next.

Shanahan announced that Pentagon comptroller David Norquist will perform the duties of deputy secretary of defense, the official said.