Five people are in critical condition after an explosion took down parts of an apartment building in Montgomery County, the U.S. state of Maryland, on Thursday morning.

One section of the four-story apartment complex, just several kilometers outside of Washington, D.C., was leveled into a pile of blackened, smoldering wreckage.

Firefighters rushed to the scene at about 10:30 a.m. (1530 GMT), and they were able to help some residents escape before the building was consumed by fire and collapsed, according to Montgomery County Fire Chief Scott Goldstein.

Goldstein also told reporters about 125 to 150 firefighters had responded to the scene, saying that "it's too early to speculate" on a cause.

Seven or eight people have less serious injuries, and some of them were transported to the hospital, Montgomery County Fire and Rescue spokesman Pete Piringer said earlier.

"This is obviously a really tragic event," Montgomery County Executive Marc Elrich said later, adding that county officials would step in to offer shelter and other aid to residents of the apartments.

Maryland Governor Larry Hogan tweeted state officials were in contact with Montgomery County authorities to offer assistance.