Rohitha Bogollagama, Sri Lanka's foreign minister, said the US authorities were trying to force Fonseka to testify "as a possible source" against Gotabhaya Rajapakse, the defence secretary and brother of Mahinda Rajapakse, the Sri Lankan president, over allegations of human rights violations.
Damien Kingsbury, a political analyst based in Melbourne, Australia, told Al Jazeera that while the US had information that war crimes were committed during the war, they do not have hard evidence.
"They would like to get some clarification from general Fonseka as to exactly what happened in the closing months of the war," he said.
"This reflects the style of the Obama administration in the US, it is going to put human rights fairly high on its international agenda."
Fonseka, who also holds US citizenship, arrived in the US last week to visit his daughters in Oklahoma.
Sri Lanka's offensive against the Tamil Tiger separatists exposed it to serious allegations of atrocities.
Human rights groups accused government forces of indiscriminate bombardments, resulting in civilian deaths. Civilian hospitals were also allegedly shelled to deprive the separatists of safe havens.
Troops killed the leader of the separatists in May, ending one of Asia's longest-running and bloodiest insurgencies.
The UN reported that more than 7,000 civilians were killed in the final few months of fighting.