Another high-ranking Pentagon officer has stepped down from his post weeks after the resignation of US defence secretary General James Mattis.

Key points: Rear Admiral Sweeney served as James Mattis's chief of staff

Rear Admiral Sweeney served as James Mattis's chief of staff His resignation followed General Mattis's departure on New Year's Eve

His resignation followed General Mattis's departure on New Year's Eve It is now the third resignation of a high-ranking Pentagon employee in a fortnight

Rear Admiral Kevin Sweeney resigned as Chief of Staff to the Secretary of Defence on Saturday, in a two-line statement posted on the department's website.

"After two years in the Pentagon, I've decided the time is right to return to the private sector," he said.

"It has been an honour to serve again alongside the men and women of the Department of Defence."

Rear Admiral Sweeney had been in the role, in which he provided counsel to General Mattis, since January, 2017.

His departure comes after General Mattis announces his resignation on December 20, days after Donald Trump declared victory over Islamic State and introduced plans to pull the US military out of Syria.

"We have defeated ISIS in Syria," Mr Trump tweeted.

General Mattis left the role of Defence Secretary not long after Mr Trump announced he was withdrawing US troops from Syria. ( AP: Jacquelyn Martin )

In his resignation letter to Mr Trump, General Mattis said the President deserved a defence secretary "whose views are better aligned" with his own.

"My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues," he said.

"We must do everything possible to advance an international order that is most conducive to our security, prosperity and values, and we are strengthened in this effort by the solidarity of our alliances."

While General Mattis had planned on remaining in the position until February 28 to allow for a "smooth transition", Mr Trump pushed for his early departure.

Just days after his resignation, Mr Trump announced General Mattis' replacement would start two months earlier than planned.

In a short farewell message sent on New Year's Eve, General Mattis told defence employees to "hold fast" and quoted a telegram Abraham Lincoln sent to General Ulysses Grant in 1865.

"Let nothing which is transpiring change, hinder or delay your military movements or plans," he said.

General Mattis was pushed out of his role two months earlier than planned. ( AP )

Deputy Defence Secretary Patrick Shanahan took over the role on an acting basis on Tuesday.

General Mattis's departure coincided with the resignation of Chief Pentagon Spokeswoman Dana W White on Monday.

She announced her intention to leave the Pentagon via the @ChiefPentSpox Twitter account, now handled by Charles E Summers Jnr, the acting assistant to the Secretary of Defence.

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Ms White was subjected to an investigation of her treatment of staff, with a report from CNN in August detailing allegations she sent employees on personal errands on her behalf, such as collecting her clothing from a drycleaner.