WEST POINT (AFP) - President Barack Obama warned Wednesday that the United States was ready to respond to China's "aggression" but said that Washington should lead by example by ratifying a key treaty.

In a wide-ranging speech on foreign policy to US military cadets at West Point, Obama said that the United States should shun isolationism and that its military must be prepared for crises.

"Regional aggression that goes unchecked - whether it's southern Ukraine, or the South China Sea, or anywhere else in the world - will ultimately impact our allies, and could draw in our military," Obama said.

But Obama emphasised caution on any decision to use force and said: "American influence is always stronger when we lead by example."

"It's a lot harder to call on China to resolve its maritime disputes under the Law of the Sea Convention when the United States Senate has refused to ratify it - despite the repeated insistence of our top military leaders that the treaty advances our national security," Obama said.

"That's not leadership; that's retreat. That's not strength; that's weakness," Obama said.

Senators of the rival Republican Party have refused to ratify the treaty, saying that the UN convention would override US sovereignty.

Tensions have been rising for months between China and its neighbours at sea, with Vietnam on Tuesday accusing Beijing of ramming and sinking one of its fishing boats in the South China Sea.

Japan and the Philippines also have tense disputes at sea with China. Japanese commentators have voiced concern that the US failure to prevent Russia from annexing Ukraine's Crimea peninsula in March sent the wrong signal to China.