The United States has delivered six coastal patrol boats to Vietnam’s coastguard in a move that expands the two nation’s security cooperation amid tension in the disputed South China Sea.

The six 45-foot Metal Shark patrol boats were officially handed over by US Ambassador Ted Osius to the coastguard Monday in the central province of Quang Nam.

The embassy said in a statement on Tuesday that the handover represented deepening cooperation in maritime law enforcement and humanitarian assistance in Vietnam’s territorial waters and exclusive economic zone.