American intelligence agencies face a significant reduction in their counterterrorism collection efforts in Africa if a proposed withdrawal of United States military forces is carried out by the Pentagon, intelligence officials said.

The new planning to pull back intelligence officers deployed in Western Africa and other parts of the continent has been partly driven by the troop deployment review, which is expected to reduce American forces in Niger, Nigeria and other countries in the region.

The presence of American troops allows intelligence officers to travel far from traditional diplomatic outposts. The troops also provide protection in the event of spreading chaos or instability. Stark evidence of the risk was seen in the lethal 2012 attacks in Benghazi, Libya, and increased security was ordered for those outposts.

If service members are soon pulled out of Africa, the C.I.A. and other intelligence agencies simply would not be able to safely deploy their officers far beyond embassy walls, officials say.

One intelligence official called the potential shift of C.I.A. officers out of Africa stunningly dangerous. The decision would not just hurt the United States’ ability to detect and stop terrorism threats, the official said, but also hinder America’s ability to collect intelligence about what rival nations, like Russia and China, are doing in Africa.

While it is difficult to assess how much of an intelligence deficit would follow a troop pullback, the loss would be real, said Nicholas J. Rasmussen, the former director of the National Counterterrorism Center.

“With a smaller military and intelligence presence, we limit how much intelligence we collect. Our analysts have a less rich pool of information on which to draw when reaching conclusions and forecasting threat conditions,” said Mr. Rasmussen, the acting executive director of Arizona State University’s McCain Institute. “Our confidence levels in the analysis we produce end up being lower.”

Trump administration officials would not say how many intelligence officers could be affected by the changes because the number of officers in the field is a closely guarded secret.

The pullback of intelligence officers is not driven only by the planned troop reductions. Counterterrorism officials are also being asked to rethink their work and narrow their focus to the most dangerous terrorist groups, according to current and former intelligence officials.

After the Sept. 11 attacks, the United States shifted resources to fighting terrorism. While most of those were focused on groups like Al Qaeda and then the Islamic State, both with the reach to orchestrate or inspire attacks on the United States homeland, American military and intelligence agencies also built up resources against regional terrorism threats.

The Trump administration, hoping to prevent the United States from becoming entangled in more long wars, wants the military and intelligence forces to scale down their ambitions. Under the plans now being discussed, fewer resources would be allocated to monitoring regional threats — terrorist groups that might spout anti-American speech but do not have the wherewithal to mount a significant attack on United States territory.

Mr. Rasmussen said no terrorist organization in Africa so far had successfully been able to attack the American homeland, giving credence to the idea that too much emphasis had been put on such groups. But without military and intelligence personnel on the ground, working with partner nations to help combat regional terrorist organizations, it becomes difficult to assess which groups have or could have the capabilities to mount an attack on the United States, Mr. Rasmussen said.

“If our intelligence picture is degraded significantly by a drawdown in presence, we run the risk of failing to collect that critical bit of intelligence that might give us insight on the capability part of the equation,” Mr. Rasmussen said.

The shift, military and intelligence officials said, is also part of an effort to move resources toward countering the rise of China and to more adroitly compete with Beijing.

But some American officials believe that cutting back the intelligence and military presence will reduce the United States’ clout in Africa at the very time it is becoming a front line in the influence battle with Russia and China.

The three nations are jostling for prominence in sub-Saharan Africa. Russia’s mercenary force, the Wagner Group, has had a presence in the Central African Republic and other countries, said Sean McFate, a professor at the National Defense University and the author of “The New Rules of War.” China has a military support base in Djibouti and is using its Belt and Road Initiative to expand its connections throughout the continent.

“Where we are competing with China is in Africa,” Mr. McFate said. “It seems shortsighted to cede the field. It is strategically myopic to move intelligence — which is the only way we are going to find out on the ground in these places — out of the region.”

Some intelligence officials insist that even if American military forces or C.I.A. officers are collecting less front-line intelligence, analysts in Washington can still draw valuable insights and warnings on terrorist threats.

But Mr. McFate said gathering knowledge about Africa was not like Eastern Europe during the years when American intelligence focused on the communist threat. A diplomat sitting in the capital simply cannot assess the strength of a terrorist group operating in a distant province, or the influence of Russian or Chinese mercenary companies.

The C.I.A. does not take policy positions in interagency discussions, and only points out the implications of different approaches. Nevertheless, inside the C.I.A. and other intelligence agencies, many experts on Africa and counterterrorism are worried that the troop pullback will have a deep impact on collection efforts, according to intelligence officials.

Even so, some officials have played down the planned pullback of troops and intelligence personnel. Without the troop presence, American officials said, they would need to switch how they collect information — relying less on officers in the field and more on intercepted communications, satellite imagery and other technical means.

But outside experts have questioned how much technical collection can compensate for a reduction in intelligence officers working in trouble spots in Africa, learning who is responsible for regional instability and what the aims of various groups are.

Mr. McFate said that as the wars in Afghanistan and Iraq wind down, there is a weariness with counterinsurgency operations, akin to the end of the Vietnam War. But pulling troops and intelligence officers out of Africa as a reaction to the exhaustion with “forever wars” is a strategic error, he said.

“America has lost an appetite for counterinsurgency; they just think of this as a never-ending war,” he said. “It is a reaction to that, but it is a strategic misstep.”