WASHINGTON (NYTIMES) - A sweeping Pentagon review of elite US commando missions is likely to result in a sharp cut - by as much as half over the next three years - in Special Operations forces in Africa, military officials said.

Ordered by Defence Secretary James Mattis in recent weeks, the assessment of Special Operations units worldwide follows an ambush in Niger that killed four US soldiers last fall. The review is an outgrowth of a Defence Department strategy that focuses on combating rising threats from Russia and China.

More than 7,300 Special Operations troops are working around the world, many of them conducting shadow wars against terrorists in Yemen, Libya, Somalia and other hot spots. The Special Operations Command in Tampa, Florida, has assumed important new missions in recent years, as well, like taking the lead on combating weapons of mass destruction.

Pentagon officials said Mattis and General Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, are worried that the commandos are spread too thin. The two leaders have ordered the military's Special Operations and Africa commands to present a range of options by mid-June to balance rising security challenges - which include North Korea and Iran - with vital counterterrorism operations.

A Pentagon investigation last month into the deadly attack in Niger exposed a risk-taking culture among commandos. That accelerated Mattis' decision to abandon some counterterrorism missions in Africa to focus on global powers, according to two Defense Department officials who spoke on the condition of anonymity.

"We need to do CT more efficiently and effectively," said Stephen Tankel, a former senior Pentagon adviser and author of a new book, With Us And Against Us: How America's Partners Help And Hinder The War On Terror.

The transition to focus on Russia and China was outlined in the Trump administration's national defence strategy released in January.

That transition has been complicated by increasing friction between Dunford, a polished, politically savvy Marine who is close to Mattis, a retired Marine general himself, and General Tony Thomas, a blunt, hard-charging Army Ranger who heads the Special Operations Command, according to five current and retired senior military officers and Pentagon officials.

The two generals have starkly different demeanors; one retired senior officer who knows both men well called Dunford "Athens" and Thomas "Sparta" to illustrate their styles of leadership.

Several officials said Dunford is frustrated with Thomas' occasional verbal gaffes.

In one example, at a Special Operations conference last month, Thomas made a teasing reference to a 2016 incident involving the Tampa mayor, who had joked that he was pretending to shoot a machine gun at the news media during the city's hostage rescue demonstrations.

In a brief interview the next day, Thomas apologised for the remarks about the news media. Later, through a spokesman, he dismissed any tension with Dunford, saying, "I'm extremely grateful for leadership who allow SOF to thrive and remain relevant to our national defence strategy," referring to Special Operations forces.

A spokesman for Dunford, Colonel Patrick Ryder, said the Joint Chiefs chairman had "utmost confidence in and respect for" Thomas, and said any suggestion that the two men were at odds was "patently false."

Even before Mattis' directive, the Special Operations Command had started relying on conventional troops to help handle some of the missions it had accumulated since the Sept. 11 attacks.

In places where Special Operations troops are frequently in combat, such as Afghanistan and Syria, regular soldiers and Marines are sometimes attached to commando units as additional security or firepower in what is known as the "uplift" program.

In the past two years, the Army has also slowly begun standing up small brigades - usually of roughly 1,000 soldiers - to take on advisory missions that are often specific to Special Forces troops and larger military units.

Soldiers are selectively recruited for those Security Force Assistance Brigades instead of taxing regular Army forces for the advisory missions.

While the Army plans to field six of the advisory units, only one is fully staffed. It is deployed to Afghanistan, where it is facing difficulties that advisory soldiers have dealt with in the past: inexperience with local troops and a stringent military bureaucracy.

At the same time, in a nod to the increasing concern about the Russian military, more Special Operations teams have been sent to the Baltic countries - Estonia, Latvia and Lithuania - and Eastern Europe to help local commando forces identify and confront possible threats from Moscow.

Nearly a decade ago, almost 13,000 Special Operations troops were deployed on missions around the globe, but a large majority were assigned to the conflicts in Iraq and Afghanistan. Of the 7,300 American commandos now operating in 92 countries, roughly half are posted outside the Middle East and South Asia, according to the Special Operations Command.

About 1,200 of those troops are on missions in Africa, and they face the most immediate likelihood of reductions. The Africa Command has been asked how it would conduct its counterterrorism missions if the number of commandos there was cut by 25 per cent over 18 months, and by 50 per cent over three years.

That would leave about 700 troops - roughly the same number as in 2014, according to data from the Africa Command's special operations branch. By comparison, there were 70 Special Operations troops on the continent in 2006.

Some of the reassigned troops could be put on potential missions against Russia or China. Or, officials said, they could rotate into deployments to ease the strain on US commandos who have repeatedly been sent abroad.

Major Sheryll I. Klinkel, a Pentagon spokeswoman, declined to comment on the specific planning reductions being considered but confirmed that the military's Joint Staff had directed the review of the Africa Command's force levels.

"In light of the National Defence Strategy's updated priorities, the Joint Staff consistently reviews plans, operations and military investments across the globe to develop the best options that address the constantly evolving threat to US national interests," Klinkel said in an e-mail.

She underscored that no final decisions had been made.

Since the deadly Oct. 4 ambush in Niger, Special Operations forces have gradually reduced the number of missions on which American advisers accompany African troops on risky operations. Those that are approved must first be vetted by officers up the chain of command who are required to take a tougher, more cautious approach when weighing the risks involved.

US commandos in Africa are now sent only on missions with local forces that are determined to have significant strategic effect, like building a new base or clearing extremists from a large area. Armed drones or other protective aircraft must accompany such missions.

If those conditions are not met, US troops will work from fortified command centers to advise African forces on intelligence, logistics, artillery and other aspects of big operations that are important - but not as prominent as front-line combat against a range of groups aligned with al-Qaeda or the Islamic State in Iraq and Syria.

A Pentagon investigation into the Niger attack found a "general lack" of "command oversight at every echelon."

Army commandos saw the findings as an attack on the culture of the Special Operations forces by high-ranking conventional military officers who did not understand the nature and demands of the job in difficult environments.