US President Donald Trump announces his strategy for the war in Afghanistan during an address to the nation from Fort Myer, Virginia.

US President Donald Trump has signed off on sending an additional 4000 troops to Afghanistan, a senior US official says.

The report came ahead of Trump's address to the nation, at 1pm Tuesday (NZT), in which he set out his strategy for Afghanistan, a conflict that has vexed his two predecessors.

Trump has long been sceptical of how the US is fighting the war in Afghanistan, which was launched by President George W Bush in October 2001 after the September 11 attacks on New York and Washington.

JOSHUA ROBERTS/REUTERS Donald Trump said the US can't afford a quick Afghanistan withdrawal.

Trump announced a strategic review soon after taking office in January and has privately questioned whether sending more troops is wise, US officials said.

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But US Defense Secretary Jim Mattis has argued that a US military presence is needed to protect against a continuing threat from Islamist militants.

Afghanistan remains one of the world's poorest countries and corruption is embedded in its politics. The Taliban is resurgent. And Afghan forces remain too weak to secure the country without American help.