A 20-year-old Tibetan nun died after she set herself on fire in Sichuan province to protest China's action in the region. Earlier this week, a former Tibetan monk attempted a similar protest suicide.
Rights group Free Tibet said in a separate incident two Tibetans were shot by Chinese security personnel during a demonstration.
Mark Toner, a spokesman for the U.S. State Department, said during his regular press briefing that Washington was "seriously concerned" by the situation.
"In the light of the continuing underlying grievances of China's Tibetan population, we would urge China and its leaders to respect the rights of Tibetans (and) to address some of the policies in these areas," he said.
Tibetan dignitaries are divided over the self-immolations, The Wall Street Journal reports.
"From the side of the Tibetan government-in-exile, it is difficult to say they should stop these self-immolations because the situation in Tibet is so desperate," monk and member of the Tibetan Parliament-in-exile Geshe Kalsan Damdul told the newspaper.