U.S. President Donald Trump has ordered the withdrawal of nearly all U.S. troops from Somalia by Jan. 15, the Pentagon said on Friday, part of a global pullback before Trump leaves office next month that will also see him draw down forces in Afghanistan and Iraq.

The United States has about 700 troops in Somalia focused on helping local forces defeat the al-Qaeda-linked al-Shabaab insurgency.

The mission has received little attention in the United States, but is considered a cornerstone of the Pentagon's global efforts to combat al-Qaeda.

A U.S. defence official, speaking on condition of anonymity, said the U.S. troops remaining in Somalia would be located in the capital, Mogadishu.

In a statement, the Pentagon sought to play down the implications of a withdrawal — experts have warned that such a pullout could have major repercussions on security in Somalia.

"While a change in force posture, this action is not a change in U.S. policy," the Pentagon said.

"The U.S. will retain the capability to conduct targeted counterterrorism operations in Somalia, and collect early warnings and indicators regarding threats to the homeland."

The Pentagon statement, which was unsigned, said some forces could be reassigned outside of East Africa. An unspecified number would be repositioned into neighbouring countries, allowing for cross-border operations, it said.

'A gift to China'

Rep. Jim Langevin, a Rhode Island Democrat, criticized the Trump pullback in Somalia as a "surrender to al-Qaeda and a gift to China." Langevin is chairman of the House armed services committee's intelligence and emerging threats and capabilities subcommittee.

"When U.S. forces leave Somalia in response to today's order, it becomes harder for diplomats and aid workers to help people resolve conflicts without violence and loss of life," Langevin said. "With upcoming elections in Somalia and conflict raging in neighbouring Ethiopia, abandoning our partners could not come at a worse time."

Langevin said China will use the opportunity to build its influence in the Horn of Africa.

The nature of the threat posed by al-Shabaab and the appropriate U.S. response has been a matter of increasing debate in the Pentagon, which has been looking for opportunities to shift its focus toward China as a greater long-term challenge.

Another U.S. official said the country's presence in Somalia was a point of contention between Trump and his former defence secretary, Mark Esper, who Trump fired last month. According to the official, Trump pushed for a drawdown citing a wish by Kenya "to police their own neighbourhood," but Esper resisted.

It is the third major withdrawal since Trump installed acting defense secretary Christopher Miller, a counterterrorism official and former Green Beret, to replace Esper at the Pentagon following Trump's election defeat.

A Defense Department watchdog report last week said U.S. Africa Command has seen a "definitive shift" this year in al-Shabaab's focus to attack U.S. interests in the region. Africa Command says al-Shabaab is Africa's most "dangerous" and "imminent" threat.