United States defence chiefs have warned of a "low but growing" probability of a war with a major power - a stark prospect for Washington, given how it also noted that other militaries are closing the gap on the United States.

In its latest National Military Strategy report, released on Wednesday, the Pentagon singled out China, Russia, North Korea and Iran as particular threats, referring to the group as "revisionist states".

The US military will shift some of its focus towards the security challenges posed by countries like Russia and China, it said, stressing that it needs to adapt its defence strategy to an increasingly uncertain global environment.

"Most states today - led by the US, its allies and partners - support established institutions and processes dedicated to preventing conflict, respecting sovereignty and furthering human rights.

"Some states, however, are attempting to revise key aspects of the international order and are acting in a manner that threatens our national security interests."

The Pentagon added: "For the past decade, our military campaigns primarily have consisted of operations against violent extremist networks. But today, and into the foreseeable future, we must pay greater attention to challenges posed by state actors. They increasingly have the capability to contest regional freedom of movement and threaten our homeland."

The increased emphasis on state actors in this year's strategic report is a marked departure from previous documents.

The last National Military Strategy report in 2011 had focused heavily on terrorist groups, like Al-Qaeda in the Arabian Peninsula, the Somalia-based Al-Shabaab and Pakistan-based Lashkar-e-Tayyiba.

The report - which outlines the strategic environment and proposes a strategy to deal with it - is released roughly once every three or four years.

While this year's report states that the US supports China's rise, it also raises strong concerns about its actions in the South China Sea.

"The international community continues to call on China to settle such (maritime) issues cooperatively and without coercion. China has responded with aggressive land reclamation efforts that will allow it to position military forces astride vital international sea lanes," said the Pentagon.

It similarly chided Russia for not respecting the sovereignty of its neighbours, and disregarding a slew of international agreements.

With the report emerging just days after negotiators had to extend the deadline on the Iran nuclear talks, the Pentagon made it clear that it is prepared to use force if a deal cannot be reached.

Said Defence Secretary Ashton Carter during a press briefing: "The basic facts have not changed recently. That is, we continue to have the tools to do (damage to Iran) and continue to maintain the military option, because the President has instructed us to, because his determination is that Iran (should) not have a nuclear weapon."

General Martin Dempsey, the chairman of the Joint Chiefs of Staff, said that the current global disorder calls for more agility, innovation and integration on the part of the military.

He pointed out in the foreword to the report that there are no longer easy fights for the US military: "Since the last National Military Strategy was published in 2011, global disorder has significantly increased while some of our comparative military advantage has begun to erode... Future conflicts will come more rapidly, last longer and take place on a much more technically challenging battlefield."