Tomislav Mercep, a wartime Croatia official who was jailed over some of the most gruesome crimes against Serb civilians during the 1990s conflict, died after suffering "a long illness", a veterans union said Tuesday.

Mercep, 68, was hospitalised earlier this month after being granted early release in March from prison, where he was serving a seven-year sentence handed down in 2017.

Mercep "has died after a serious and long ilness," said the Association of Croatian Volunteers of the Homeland War, a veterans group which he became president of in 1995.

An interior ministry official during the war, Mercep was found guilty by a Zagreb court in 2016 of tacitly approving the killing, illegal detention, inhuman treatment and looting of property of 52 ethnic Serbs by members of a unit under his authority.

The court in Zagreb sentenced him to five and a half years in jail before the Supreme Court sentenced him the following year to seven years in prison.

At least 43 of his victims died. 

They included a Serb couple from Zagreb with a 12-year-old daughter, who were shot dead in cold blood in the capital in late 1991.

Apart from the Zagreb region, crimes were committed also in eastern Croatia.

Mercep pleaded not guilty during his trial.

The 1991-95 conflict was sparked after Croatia declared independence from former Yugoslavia, sparking a war with Belgrade-backed rebel Serbs who opposed the move.

The conflict claimed some 20,000 lives.