Aid groups warned of a humanitarian crisis unfolding with air and sea blockades making it impossible to send desperately needed assistance as casualties mount.

In the heaviest raids yet of the six-day Saudi-led air war against Shiite Huthi rebels, explosions lit up the skies over Sanaa on Monday night and rocked other parts of the country on Tuesday.

Amnesty International accused the Saudi-led coalition of “turning a blind eye to civilian deaths” as it reported four children were among six burned to death in strikes on Ibb, in central Yemen.