Washington is reportedly pushing British Prime Minister Theresa May to increase the number of UK troops based in Afghanistan. It comes as President Trump announced US policy in the country which has been the scene of the longest war in its history.

"Britain as one of the big military players in the alliance is expected to back the US with additional troops but the pressure is on everyone in the alliance," a diplomatic source told The Times. "The concept of all for one and one for all looks a bit thin if it's the Americans who have to do everything."

The paper reported that UK defence sources had denied the UK was under any pressure from Washington.

On Monday night (21 August), Trump had been expected to announce a troop increase during a prime time address from the Fort Myer military base in Virginia on but did not specify any numbers.

Trump said: "Our nation must seek an honourable and enduring outcome worthy of the tremendous sacrifices that have been made."

The US president added that the consequences of a rapid exit would be "predictable and unacceptable" and that while initially he wanted to pull US forces out, he now wanted to avoid the mistakes made in Iraq.

"A hasty withdrawal would create a vacuum that terrorists, including ISIS and al Qaida, would instantly fill just as happened before September 11."

Trump said he would "not talk about numbers of troops or plans for further military activities".

His statement comes following a lengthy review with his administration on a new strategy for the US war in Afghanistan, The Washington Post reported.

There are currently 8,400 US troops serving in Afghanistan, down from 100,000 at the height of the conflict. But Trump said the focus of US policy would be help Afghanistan defend itself.

"The stronger the Afghan security forces become, the less we will have to do. We want them to succeed but we will no longer use American military might to try to construct democracies in far off lands," he told the audience in Arlington.

Earlier, US Defence Secretary James Mattis said the administration's new strategic focus would be expanded to resolve America's longest war. "It is not just an Afghanistan strategy," Mattis said. "If you look at the region, it's a south Asia strategy, and we'll be addressing those issues in it."

According to The Times, the US prefers working with the UK because of their long military relationship. Mattis and UK Defence Secretary Sir Michael Fallon also have a close working relationship.

Britain has offered to deploy 85 troops by November, raising the total number of British troops to 585. It will also examine whether it can plug "capability gaps" and could offer aircraft and logistical support, The Times reported. However, Britain is reportedly reluctant to send additional troops.