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The US defence secretary has denied claims that the US is planning to pull troops from Iraq amid mounting tensions with Iran.

A leaked letter had suggested that US-led coalition fighting against Islamic State would withdraw.

A push led by pro-Iran factions to oust US troops from Iraq had followed the US airstrike that killed a top Iranian general Qasem Soleimani​.

However, defence secretary Mark Esper said: "There's been no decision whatsoever to leave Iraq."

It comes as the UK began reducing staff at its embassies in Iran and Iraq to a minimum level amid security concerns and as Boris Johnson prepared to chair a National Security Council meeting amid the growing crisis.

Details of a letter said to be sent from United States Marine Corps Brigadier General William H Seely III, the commanding general of Task Force Iraq said forces would be leaving Iraq.

The letter seen by Reuters said: "Sir, in deference to the sovereignty of the Republic of Iraq, and as requested by the Iraqi Parliament and the Prime Minister, CJTF-OIR will be repositioning forces over the course of the coming days and weeks to prepare for onward movement."

The authenticity of the letter, which was addressed to the Iraqi defence ministry's Combined Joint Operations Baghdad, was confirmed to Reuters independently by an Iraqi military source.

"We respect your sovereign decision to order our departure," it said.

The letter said: "During this time, there will be an increase in helicopter travel in and around the International Zone (IZ) of Baghdad."

Iran's demand for US forces to withdraw from the region gained traction on Sunday when Iraq's parliament passed a resolution calling for all foreign troops to leave the country.

Iraqi caretaker Prime Minister Abdel Abdul Mahdi told the US ambassador to Baghdad on Monday that both nations needed to implement the resolution. However, no timeline was given.

Iraq has officially condemned the US airstrike that killed Iran's top military commander as a "flagrant violation" of the terms of the American forces' presence in the country.

In a letter to the UN Security Council, Iraq's UN ambassador Mohammed Hussein Bahr Aluloom called on the council to condemn the US airstrike and "assassination" that killed Soleimani and a senior Iraqi militia commander.

He said the airstrike was "a dangerous escalation that might ignite a devastating war in Iraq, the region and the world", and urged the council to hold accountable "those who have committed such violations".

Bahr Aluloom also called on the council to ensure Iraq is not "dragged into international and regional crises" and to prevent "the law of the jungle" from prevailing.

However any security council action sought by Iraq against the Trump administration is virtually certain to be vetoed by the US.

Elsewhere, Britain has reduced staff at its embassies in Iran and Iraq to a minimum level amid escalating tensions on the Persian Gulf.

The withdrawal of the diplomats is a precautionary step rather than based on specific intelligence of a threat, according to Sky News.

Ambassadors Rob Macaire in Tehran and Stephen Hickey in Baghdad will remain in place, the news site reported.

The Foreign Office said its embassies in Baghdad and Tehran remained open but declined to comment on operational detail.

"The safety and security of our staff is of paramount importance and we keep our security posture under regular review," a spokesman said.