Fresh gunfire erupted Tuesday in downtown Port-au-Prince, forcing aid workers to halt urgently needed care for thousands of Haitians.

Weeks of gang violence have forced some 18 hospitals to stop working and caused a shortage of medical supplies as Haiti’s biggest seaport and main international airport remain closed, warned aid workers with The Alliance for International Medical Action, a Senegal-based humanitarian organization.

“The situation is really challenging and affects our movement on a daily basis,” said Antoine Maillard, the organization’s medical coordinator based in Port-au-Prince.