The head of US intelligence has suggested China is becoming more cautious about military action against Taiwan, following reports that Russian troops are facing strong resistance in Ukraine.

Director of National Intelligence Avril Haines told the Senate Armed Services Committee on Tuesday that the Chinese are working hard to enable their military to take Taiwan despite US intervention.

But she added that China hopes to unify with Taiwan without using force, so it is stepping up diplomatic, economic and military pressure.

Haines noted that China has been analyzing Russia's invasion of Ukraine.

She said China was "surprised by the degree to which the United States and Europe came together to enact sanctions, and that is something they obviously will be looking at in the context of Taiwan."

Haines added that seeing what happened to Russia may give the Chinese less confidence about what is likely to happen in a Taiwan invasion.