PHIL STEWART and DAVID LEWIS  THE United States believes Eritrean soldiers have crossed into Ethiopia, despite denials from both nations, a U.S. government source and five regional diplomats said, as Prime Minister Abiy Ahmed's government battles a rebellious northern force. Abiy and Eritrea's President Isaias Afwerki signed a peace pact ending two decades of hostilities in 2018 and now regard the Tigray People's Liberation Front (TPLF) as a mutual foe. The U.S. assessment creates a potential policy predicament as Washington views Ethiopia as a major ally in the volatile Horn of Africa but accuses Eritrea of severe rights abuses. Eritrean…