NAIROBI (Reuters) - The U.S. military may be guilty of war crimes for killing large numbers of civilians in a sharply stepped-up campaign of air strikes in Somalia over the past two years, Amnesty International said.

The rights group said it had been able to document 14 civilians killed in investigations of just five air strikes, a tiny fraction of at least 110 such strikes that the U.S. military says it has launched since June, 2017.

The U.S. military rejected Amnesty’s report. It says it has killed 800 militants in air strikes in Somalia over that period, but has not wounded or killed a single civilian.

“We currently assesses no civilian casualties have occurred as a result of any U.S. Africa Command air strikes,” the U.S. military’s Africa command AFRICOM said in en emailed response to Reuters.

Brian Castner, Amnesty International’s Senior Crisis Advisor on Arms and Military Operations, said the civilian death toll in the small number of air strikes the rights group was able to investigate suggested that the “shroud of secrecy surrounding the U.S. role in Somalia’s war is actually a smokescreen for impunity”.

“Members of the U.S. government forces who planned and carried out the air strikes may have committed violations of international humanitarian law, including unlawful killings, which could amount to war crimes,” Amnesty’s report said.

Somalia, one of the poorest countries in the world, has been in a state of civil war and profound insecurity since 1991. In recent years, the U.S. military has been supporting a U.N.-backed government in Mogadishu fighting against an insurgency by the Al Shabaab Islamist militant group.

In March 2017, President Donald Trump gave the military greater authority to carry out strikes and raids in Somalia, including without waiting for militants to attack U.S. allies.

In a statement, AFRICOM, said: “We have processes in place to ensure the safety and protection of the local population remains a top priority. These procedures, combined with precision strike capabilities, safeguard civilians and infrastructure.”

A U.S. air strike this week killed four people -- an employee of mobile phone company Hormuud Telecoms and three unidentified passengers -- a relative of one of the victims told Reuters on Tuesday.

AFRICOM said it had killed three militants in an air strike on Monday, adding it was aware of reports alleging civilian casualties and would review the information about the incident.

Amnesty’s report investigated five air strikes in Lower Shabelle region. It concluded that 14 civilians had died and eight were injured.

Al Shabaab was pushed out of the capital Mogadishu in 2011, but retains a strong presence in parts of southern and central Somalia. The militants said U.S. attacks inflict damage on local residents and encourage relatives of victims to join them.

“U.S. strikes target farmers and pastoralists many times in many places of Somalia. People and their farms and animals perish. Their houses get burned,” Abdiasis Abu Musab, an Al Shabaab spokesman, told Reuters on Tuesday.