By Eric M. Johnson SEATTLE (Reuters) - Magma levels are slowly rebuilding inside Mount St. Helens, a volcano in Washington state that erupted in 1980 and killed 57 people, although there was no sign of an impending eruption, U.S. scientists said. "The magma reservoir beneath Mount St. Helens has been slowly re-pressurizing since 2008," the U.S. Geological Survey said in a statement on Wednesday. "It is likely that re-pressurization is caused by (the) arrival of a small amount of additional magma 4 to 8 km (2.5 to 5 miles) beneath the surface." The USGS said this is to be expected with an active volcano and does not indicate "the volcano is likely to erupt anytime soon." The USGS, and the Pacific Northwest Seismic Network at University of Washington, closely monitor ground deformation and seismicity at the volcano.