A US appeals court has upheld a ruling that blocks schools in the state of Massachusetts from teaching literature that denies the mass killing of Armenians in Turkey in 1915 was a genocide.

The ruling came in response to a 2005 lawsuit filed by the Assembly of Turkish American Associations, a US lobbying group. A lower court dismissed the suit in June, and the appeals court upheld that decision on Wednesday.

State curriculum in Massachusetts requires schools to teach a unit about the Armenian genocide, the Holocaust, and other "recognised human rights violations and genocides."

The appeals court ruled that "law would not allow the genocide denial actions that the plaintiffs sought."

Turkish-American groups have lobbied schools to include materials that question whether the 1915 killings were, in fact, a genocide.

Historians have concluded that more than one million Armenians were killed in 1915, and contemporaneous reports from newspapers and foreign embassies often called the killings "systematic."

The International Association of Genocide Scholars affirmed in 2005 that the killings were a genocide.

The Turkish government claims the death toll is far lower, and that the killings were part of broader civil unrest, not a planned extermination.

Labelling the Armenian genocide has long been a touchy subject in the US. The foreign affairs committee of the US house of representatives voted narrowly earlier this year to declare the killings a genocide, a move that prompted Turkey to recall its ambassador to the US.

Massachusetts has one of the largest Armenian populations in the US.