June 29 (UPI) -- Extreme heat that has been baking Europe over the last two weeks is at least partially responsible for hundreds of additional deaths, according to the United Nations' health chief. The continent has been gripped by a heatwave that has sent temperatures soaring from the high 90s to low 100s Fahrenheit since at least June 17, when Britain's Met Office warned that a significant heatwave was developing across parts of Europe. Since then, a number of European nations, including