More than 130 retired diplomats and other former senior U.S. officials issued an open letter on Thursday criticising a planned overhaul of the State Department that could see thousands of employees laid off.

"We strongly condemn Secretary of State Marco Rubio’s announced decision to implement sweeping staff reductions and reorganisation at the U.S. Department of State," the officials said in the letter.

The signatories included dozens of former ambassadors and senior officials, including Susan Rice, who served as national security advisor under President Barack Obama, a Democrat.

The timing of the cuts remains unclear, with the U.S. Supreme Court expected to weigh in at any moment on a bid by U.S. President Donald Trump's administration to halt a judicial order blocking the firings.

The administration in late May notified Congress of a plan to overhaul its diplomatic corps that could cut thousands of jobs, including hundreds of members of its elite Foreign Service who advocate for U.S. interests in the face of growing assertiveness from adversaries such as China and Russia.

Initial plans to send the notices last month were halted after a federal judge on June 13 temporarily blocked the State Department from implementing the reorganisation plan.

The shake-up forms part of a push by Trump to shrink the federal bureaucracy, cut what he says is wasteful spending, and align what remains with his "America First" priorities.

"At a time when the United States faces unprecedented challenges from strategic competitors, ongoing conflicts, and emerging security threats, Secretary Rubio’s decision to gut the State Department’s institutional knowledge and operational capacity is reckless," the former officials wrote.