Eyewitnesses say Ethiopian soldiers have crossed the border into Somalia in significant numbers. They report seeing at least 20 vehicles carrying troops and a few hundred soldiers in the Galgudud region of central Somalia.
Ethiopia withdrew its troops from the country in 2009 after a controversial three-year presence.
Kenyan soldiers are already in southern Somalia. They are involved in struggle to defeat fighters from the Islamist group al-Shabab who have made several incursions into Kenya mainly to kidnap European nationals.
Ethiopian government sources have so far denied the latest reports but if they turn out to be true it will be the largest deployment of their troops to Somalia since the withdrawal. Their presence in 2009 proved unpopular with ordinary Somalis.