The number of UK troops in Afghanistan is set to double after Donald Trump pressured his international allies to do more to combat the Taliban.

Between 400 to 450 soldiers could be sent to the country which would bring the total UK deployment to more than 1,000.

The US has increased its presence in Afghanistan since Mr Trump unveiled his South Asia strategy in August last year.

Around 4,000 troops are thought to have been added to the 8,400-strong US deployment in the country, to bolster the Kabul government, train Afghan forces and fight the Taliban and other militant groups.

And now Mr Trump is expecting the other NATO members to do their share after strongly criticising those allies like Germany who don't spend the required 2% on defence.

He will ramp up the pressure ahead of the NATO summit in July with the ultimate threat that the US, which is the biggest contributor, could withdraw from the alliance which would in effect kill it off.

Donald Trump is putting pressure on other NATO members to do more (
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REX/Shutterstock)

Defence Secretary Gavin Williamson is understood to have written to the Prime Minister recommending the increase, although a decision is yet to be made.

The Ministry of Defence said the UK's contribution to the mission was kept "under constant review".

But former general Sir Richard Barrons warned that even more troops could be required.

The majority of troops currently there are in a training capacity but Sir Richard warned they should be prepared to accompany Afghan forces on operations.

The retired officer said the decision to end combat operations in Afghanistan at the end of 2014 "hasn't worked".

He told BBC Radio 4's Today that 400 extra troops would "send an important message to our allies" that they should do more and would also signal to the Taliban "they will never bring this fight in Afghanistan to an end by fighting, they have to resort to dialogue".

But he suggested a greater military presence and an enhanced - and more dangerous - role should be considered, with troops leaving the relative safety of their bases to accompany Afghan forces on operations.

"I absolutely think, if we are going to make a meaningful contribution, we will have to find the courage to train, advise, assist and accompany them into action," Gen Barrons said.

It is understood that Gavin Williamson has written to the PM (
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"With that comes some risks and consequences, but that's how you make a difference."

Gen Barrons suggested that the UK should deploy around 10% of the troops sent by the US, which would lift the British contribution to more than 1,200.

A Ministry of Defence spokesman said: "The support the UK provides Afghanistan on security, development and governance is crucial to building a stable state and reducing the terrorist threat to the UK.

"We remain committed to Nato's non-combat Resolute Support mission, in which we play an important role, and keep our contribution under constant review."

The UK decided to end military operations in Afghanistan in 2014 (
Image:
PA)

The military uplift is intended to prevent a Taliban takeover of the country, target Islamic State and al Qaida.

On Thursday, US State Department spokeswoman Heather Nauert said secretary of state Mike Pompeo had "reinforced our enduring investment in Afghanistan" in a conversation with the country's president, Ashraf Ghani.

The reported change in approach is said to come amid concerns over how Mr Trump will approach a coming meeting of Nato leaders.

Theresa May is expected to join the president at the summit in Brussels in July, with Mr Trump then set to make a visit to the UK.

Mr Trump has repeatedly called for Nato members to meet their defence spending commitment of 2% of gross domestic product (GDP) and has threatened to walk away from the organisation if his demands are not met.

During a meeting with Mr Trump in Washington on Thursday, Nato general secretary Jens Stoltenberg said all members have increased their spending, in part due to Mr Trump's approach.

After the talks Mr Stoltenberg said it is "very important that we all contribute more to our shared security".