This is the first time regular US troops have been deployed in Somalia since 1994, although some counter-terrorism advisers are already there.
President Donald Trump last month approved a directive allowing tougher action against al-Shabab.
In 1993, 18 US special forces personnel were killed in the incident dramatised in the Hollywood film Black Hawk Down.
The deaths, and the shooting down of two US helicopters in Mogadishu, shocked the US and the rest of its military personnel were withdrawn from Somalia shortly afterwards.
Hundreds of Somalis were also killed in the 15-hour battle sparked when US forces tried to capture close allies of warlord Mohammed Farah Aideed.
Since then, the US has restricted most of its activities in Somalia to drone and missile attacks against Islamist militants.
The main focus now is the Somali army, which is fractured, undisciplined and poorly equipped, she says.
Several other countries, including the UK and Turkey, are also training Somali troops.
Al-Shabab, part of al-Qaeda, has a strong presence in many rural parts of Somalia and often stages attacks in the capital, Mogadishu, and elsewhere.
The African Union has a force of about 22,000 soldiers helping the Somali government fight al-Shabab.