Obama denies US power has ebbed

President Barack Obama has mounted a defiant defence of his global leadership, rebuking critics who see him as weak, but warning that not every global threat justifies a US military response.

In a major speech at the West Point military academy on Wednesday, Obama denied US power had ebbed under his watch, after he withdrew troops from Iraq and as he does the same in Afghanistan.

He also pledged to ramp up support for Syrian rebels, vowed to stand up to Russia over Ukraine and promised to make drone strikes against terror suspects more transparent.

He vowed to hold China accountable to international "rules of the road" in the South China Sea and elsewhere.

"To say that we have an interest in pursuing peace and freedom beyond our borders is not to say that every problem has a military solution," Obama said.

"Since World War II, some of our most costly mistakes came not from our restraint, but from our willingness to rush into military adventures, without thinking through the consequences," Obama said, in an apparent reference to the Iraq war, which he has branded a disaster.

The president's speech came with his foreign policy, which was once seen as a political asset, under assault from critics who believe he is being outmanoeuvred by strongmen like Russian President Vladimir Putin and China's President Xi Jinping.

"Here's my bottom line: America must always lead on the world stage. If we don't, no one else will. The military ... is, and always will be, the backbone of that leadership."

Obama was particularly exercised by those who complain he should have deployed the US military in Syria or made a more robust strategic response to Russia's annexation of Ukraine, or who complain that he has left Iraq or Afghanistan to fend for themselves.

"Tough talk often draws headlines, but war rarely conforms to slogans," Obama said.

However, he added US military action cannot be the only, or even primary, component of US leadership in every instance.

"Just because we have the best hammer does not mean that every problem is a nail," Obama told a graduation ceremony at the college.

"And because the costs associated with military action are so high, you should expect every civilian leader - and especially your commander in chief - to be clear about how that awesome power should be used."

Obama said he was "haunted" by the deaths of American servicemen under his watch -- including some who attended previous commencement ceremonies he had given at West Point.

The president has been under increasing pressure to do more to support Syrian rebels battling President Bashar al-Assad and offer some relief to civilians reeling under strikes including barrel bombings carried out by the regime.

He also said that terrorism remained the biggest national security threat to the US and unveiled a new $US5 billion ($A5.41 billion) fund to equip and train allies on the front lines of the struggle against terrorism, for instance in Africa.