US President Barack Obama says he's advised Donald Trump to "think it through" before making any changes to the US approach to the "one-China" policy. 

"The idea of One-China is at the heart of their conception as a nation. And so if you are going to upend this understanding, you have to have thought through what the consequences are because the China and the Chinese will not treat that," Obama said.

The US agreement with Beijing on the "one-China" policy became an issue after Trump broke decades of diplomatic protocol by having a telephone conversation with the Taiwan leader. Obama says he advised Trump to have his full team in place and be fully briefed on issues before considering any changes.

The out-going president also added at his final year-end news conference that the Taiwan issue is of utmost importance to the Chinese, and how they react to any potential US policy change could be very significant.