NAIROBI, March 29 (Reuters) - Kenya charged two men on Thursday with killing two American women riding in a U.S. Embassy vehicle that was carjacked near Nairobi in January. The two suspects, Simon Wanaina and Paul Kariuki, denied the charges before the chief magistrates' court in the capital. Police say men with AK-47 rifles shot dead the women, both relatives of a U.S. diplomat, before stealing their car. Wanaina and Kariuki face execution if convicted. Their trial was set for May 5. The U.S. Embassy declined to comment. Both men appeared in court this week charged with killing a prominent HIV/AIDS researcher during a separate robbery in February. They and a third man denied those charges. Carjackings of Kenyans and foreigners are common in Nairobi. Rapes are also common during the attacks. The U.S. government warned its citizens last month to "consider carefully the risks of travel to Kenya" following a spate of violent crimes that included four attacks on diplomats or their families in less than a year.