Sources are now reporting the second bus explosion in Russia in 24 hours has killed more than earlier seen. At least 15 were killed in the blast, which hit Volgograd (the former Stalingrad). The explosion rocked a trolley in the city and is an escalation by terrorists most likely affiliated with Chechen rebels.
There is a live feed at Russia Today, which is not normally a reliable source. Still, there is valuable information here.