US Defense Secretary James Mattis on Wednesday outlined initial plans to further grow America's vast military, after President Donald Trump vowed a "great rebuilding" of the US armed services.

In a short memo, Mattis highlighted critical areas where the Pentagon could immediately use some extra cash.

He proposed an amendment to this year's budget that would free up money to address shortfalls in military readiness and to fund "new requirements" from the "acceleration" of the fight against the Islamic State group.

Trump last week signed an executive order to begin increasing the size of the US military, promising new aircraft, naval ships and more resources for the Pentagon.

The order provided few specifics but Trump made clear during last year's campaign and in recent weeks that he wants to enlarge the ranks of the military services and expand its fleets of aircraft and ships.

For example, he has said he envisioned a naval fleet of 350 vessels, up from the navy's current 274 and more than its 310 vessel target.

The US military is already by far the world's most powerful and most expensive -- with bases spanning the globe, an annual budget of more than $600 billion and about 1.3 million active-duty troops.

For next year's budget, Mattis said the Pentagon is conducting a comprehensive review that among other things will focus on buying more munitions, funding Defense Department facilities at a higher rate and "growing force structure" as fast as possible.

After the withdrawal of most US troops from Iraq and Afghanistan, the administration of Barack Obama saw a shrinking Pentagon budget but Republicans have long insisted cuts went too far and hurt US capabilities, especially in the face of Chinese and Russian military expansions.