The Libyan coast guard intercepted a humanitarian rescue ship in the Mediterranean on Tuesday, ordering it to sail to Tripoli or risk being targeted, a Reuters photographer aboard said.

It was the latest in a series of incidents in which nongovernmental organizations (NGOs), operating on the edge of Libyan waters to rescue migrants trying to cross from Africa to Italy, have been threatened by the Libyan coast guard.

The Golfo Azzurro, operated by Spanish humanitarian organization Proactiva Open Arms, was repeatedly contacted by a Libyan coast guard vessel, according to radio transmissions heard by the photographer.