More than 1,300 US State Department employees were fired on Friday in a downsizing ordered by President Donald Trump and touted as cutting bloated government.

Diplomats and other staff clapped out departing colleagues in emotional scenes at the Washington headquarters of the department, which runs US foreign policy and the global network of embassies.

Some were crying as they walked out with boxes of belongings.

A State Department official said 1,107 members of the civil service and 246 Foreign Service diplomatic employees were terminated.

The layoffs at the department came three days after the US Supreme Court cleared the way for the Trump administration to begin carrying out its plan to gut entire government departments.

The conservative-dominated top court lifted a temporary block imposed by a lower court on Trump's plans to lay off potentially tens of thousands of employees.

The 79-year-old Republican says he wants to dismantle what he calls the "deep state." Since taking office in January, he has worked quickly to install fierce personal loyalists and to fire swaths of veteran government workers.

His secretary of state, Marco Rubio, says the foreign policy department is too cumbersome and requires thinning out of some 15 percent.

The American Foreign Service Association (AFSA) – the union representing State Department employees – condemned the "catastrophic blow to our national interests."

"At a moment of great global instability – with war raging in Ukraine, conflict between Israel and Iran, and authoritarian regimes testing the boundaries of international order – the United States has chosen to gut its frontline diplomatic workforce," AFSA said in a statement.

"We oppose this decision in the strongest terms."

The US State Department employed over 80,000 people worldwide last year, according to a fact sheet, with around 17,700 in domestic roles.

The US Agency for International Development, long the primary vehicle to provide US humanitarian assistance around the world, has already been mostly dismantled.

According to The Washington Post, State Department employees were informed of their firings by email.

Foreign Service officers will lose their jobs 120 days after receiving the notice and will be immediately placed on administrative leave, while civil service employees will be separated after 60 days, the newspaper said. (AFP)