Trump set to outline new US strategy for Afghanistan

"He wants to be the one to announce it to the American people," Mattis said on Sunday in reference to the decision on Afghanistan.

"He now needs the weekend to collect his thoughts on how he's going to explain it to the American people," he added.

In a tweet posted on Saturday, the president said that Afghanistan was one of the issues discussed with top Trump administration officials at Camp David on Friday.

Trump's Afghanistan strategy has been the subject of deliberations for several months now, with Mattis having previously said that the strategy would be ready by mid-July.

The strategy review that has taken place has seen several several suggestions presented to top officials. This included a presentation from Republican Senator John McCain, who proposed increasing numbers of US troops and airstrikes.

Erik Prince, founder of security company Blackwater, urged the Trump administration to shift its strategy toward greater private sector involvement.

The new strategy Trump is set to announce will be the latest of many policy shifts in Afghanistan, where the US has been heavily involved since 2001.

Despite former President Barack Obama brought combat operations to an end in 2014, however, the US has continued to support the Afghan government against Taliban and Islamic State group insurgents.

A recent spike in violence and the growing strength of the militants has led US policy makers to consider a "surge" strategy to counter the Taliban and IS insurgency.