Military leaders warned US lawmakers Tuesday that aging equipment, underfunding and understaffing have left the Pentagon critically unprepared, as the Republican-led Congress hopes to usher in a spending bonanza under President Donald Trump.

The president has repeatedly said the vast US military is suffering from massive shortages and has vowed a "great rebuilding" of the armed services.

Many officials worry that budget caps, known as sequestration, implemented under Barack Obama, have left the military depleted and lacking the readiness to deal with the full array of potential global threats.

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.

Republicans are keen to spend billions more, though it is not clear where the extra cash would come from, especially if the Trump administration cuts taxes.

"I continue to be concerned... by evidence that is accumulating on the damage inflicted upon our military in recent years and the stresses on the force," said Republican Congressman Mac Thornberry, who chairs the House Armed Services Committee.

"With a new administration and a new Congress, we have the opportunity to begin the repairs," he added.

Trump on Monday told leaders at US Central Command they would get "beautiful new planes and beautiful new equipment."

"You are going to get a lot of equipment. Believe me," he said.

General Stephen Wilson, vice chief of staff of the Air Force, told lawmakers that potential US adversaries have been modernizing their forces to compete with the United States.

China and Russia "have watched our fighting and they are preparing their forces. We need to prepare to fight any adversary," Wilson said while depicting an Air Force suffering from a shortage of pilots and aging planes.

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

He has offered few specifics but has said he envisioned a naval fleet of 350 vessels, up from the navy's current 274 and more than its 310-vessel target.

For next year's budget, Pentagon chief Jim Mattis has ordered 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, Obama oversaw 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.