US President Donald Trump has said he has fired his national security adviser John Bolton, saying he disagreed "strongly" with his positions.

"I asked John for his resignation, which was given to me this morning," Mr Trump said on Twitter.

"I informed John Bolton last night that his services are no longer needed at the White House," Mr Trump wrote, saying he would name a replacement next week.

Mr Bolton himself disputed Mr Trump's version of events, apparently saying that the president had not fired him in person, as he claimed.

"I offered to resign last night and President Trump said, "Let's talk about it tomorrow," Mr Bolton tweeted.

The news - coming days after Mr Trump caused uproar by revealing he was cancelling secret talks with Afghanistan's Taliban - stunned Washington.

According to US media reports Mr Trump's extraordinary but failed bid to fly Taliban leaders into the presidential retreat at Camp David last weekend sparked a major, final row.

Mr Bolton is a veteran and controversial figure closely linked to the invasion of Iraq and other aggressive foreign policy decisions.

He had been seen as one of the main driving forces in the White House's tough approach to Iran, Venezuela and other trouble spots.

Secretary of State Mike Pompeo cautioned that Mr Bolton's exit should not be interpreted as heralding strategy changes.

"I don't think any leader around the world should make any assumption that because someone of us departs that President Trump's foreign policy will change in a material way," he told reporters.

Treasury Secretary Steven Mnuchin underlined that Mr Trump and top aides remain "completely aligned" on Washington's crippling sanctions against Iran, known as the maximum pressure campaign.

But when asked if Mr Trump was still open to meeting his Iranian counterpart Hassan Rouhani at the United Nations General Assembly this month - an event that would be as ground breaking as his proposed Taliban talks - Mr Pompeo said "sure."

As often in the Trump presidency, the abrupt reshuffle appeared to contain an element of chaos.

Mr Trump's tweeted announcement came shortly after the White House press office had said Mr Bolton would shortly be giving a press conference on terrorism issues alongside Mr Pompeo.

Mr Bolton joins a growing list of senior officials to have come and gone during the Republican businessman's tumultuous first term in office.

Since entering the White House in January 2017, Trump has had two secretaries of defence, as well as two acting secretaries, two secretaries of state, two CIA directors and a half dozen communications directors.