Secretary of State Rex Tillerson defended plans to slash his own department's budget Tuesday, arguing Washington will need to spend less on diplomacy as armed conflicts come to an end.

Tillerson's effort to cut the $55 billion State Department budget by up to a third has been rejected by Congress, mocked in the media and even privately attacked by senior serving diplomats.

But he insists that he is merely trying to contain and manage what had become a historically high spend, while streamlining procedures and building a more effective US diplomatic corps.

There is an element of optimism in his plan too, however, as it seems predicated on the United States finally being able to disentangle itself from long-running wars in Asia and the Middle East.

"Part of this bringing the budget numbers back down is reflective of an expectation that we're going to have success in some of these conflict areas, of getting these conflicts resolved and moving to a different place in terms of the kind of support that we have to give them," he said.

US forces have been fighting in Afghanistan since 2001 and Iraq since 2003, and thousands of troops have since deployed in growing numbers to pursue Islamist militants in Syria and parts of north and west Africa.

Alongside the larger military footprint, US diplomats have been involved in negotiating ceasefires, maintaining coalitions, promoting peace talks and helping local officials rebuild war-shattered administrations.

This helped drive the State Department budget up to a level which Tillerson argued Tuesday is "just not sustainable" and ought now to fall back to "the mid-30 billion level."