Under the new guidelines, Africa Command may treat Somalia under less-restrictive battlefield rules: Without interagency vetting, commanders may strike people thought to be Shabab fighters based only on that status, without any reason to think that the individual target poses a particular and specific threat to Americans.

In addition, some civilian bystander deaths would be permitted if deemed necessary and proportionate. Mr. Trump’s decision to exempt much of Somalia from the 2013 rules follows a similar decision he made for parts of Yemen shortly after taking office.

The new directive for Somalia is another example of how the American military is accelerating the ways it carries out combat missions under the Trump administration, reducing constraints on the use of force imposed by the Obama administration.

As the fight against the Islamic State in Iraq and Syria has recently moved into the city of Mosul, civilian casualties have spiked. One American strike on March 17 may have killed scores of civilians, and human rights groups have questioned whether the rules of engagement were to blame.

While American commanders say the formal rules of engagement have not changed in Iraq, they acknowledge that the system for calling in airstrikes there has been accelerated. Gen. Joseph L. Votel, the commander of United States Central Command, said on Wednesday that the new procedures made it easier for commanders in the field to call in airstrikes without waiting for permission from more senior officers.

The loosening of the rules in Somalia comes against the backdrop of a broader, continuing Trump administration policy review about whether to scrap the 2013 rules altogether. The decision was described by officials familiar with the new directive who spoke on the condition of anonymity to discuss military planning.

Luke Hartig, a former senior director for counterterrorism at the National Security Council during the Obama administration, said greater action could be helpful in dealing with a threat, pointing to the Obama administration’s decision last year to temporarily declare the region around Surt, Libya, an active-hostilities zone. That decision similarly permitted airstrikes that helped Libyan forces root out Islamic State militants.