BEIJING  A Tibetan exile group in India says that the Chinese authorities have executed four people convicted for their roles in the riots that convulsed Tibet last year.

According to the Tibetan Center for Human Rights and Democracy, the four were put to death on Tuesday, more than six months after they were tried and convicted of starting fires in the Tibetan capital, Lhasa, that killed seven people.

At least 18 people died in March 2008 during violence that was directed at Han Chinese migrants, whose growing presence in the region has angered many native Tibetans. Since then at least 84 people have been convicted during trials that rights groups say are opaque, cursory and unfair.

The executions were not announced by the Chinese news media, and a woman who answered the phone at the Lhasa Municipal Intermediate People’s Court hung up when asked to confirm the accounts provided by the exile group.