The United States plans to send 3,000 more troops to Afghanistan, Defence Secretary James Mattis said Monday, four weeks after US President Donald Trump announced a new strategy for Afghanistan and South Asia.

Some of the troops are already on their way to the country. Trump announced the strategy last month following an increase in attacks on district centres in many parts of the country carried out by Taliban militants.

Trump said the objective of the new strategy for Afghanistan and South Asia makes hunting down and killing terrorists its top priority.

The United States has had troops deployed in Afghanistan since shortly after the September 11 terrorist attacks in 2001, making it the US military's longest conflict.

In announcing the strategy in August, the president did not specify the number of troops he would send, but an increase of 4,000 troops had been cited, according to news reports quoting unidentified congressional officials who had been briefed by the administration.

The US military currently has some 11,000 soldiers in Afghanistan. The Taliban controls about 11 per cent of the country, while another 30 per cent is contested. Daesh terrorist group is also present in the country.

The US has deployed military trainers in Afghanistan within the framework of the NATO mission Resolute Support.

It also conducts airstrikes independent of NATO under its own combat mission called Operation Freedom Sentinel, and has special forces on the ground.