Foreign crews are growing increasingly angry at mounting restrictions placed on their shore leave in US ports in the name of maritime security, a leading advocate says.

"They're pissed," said Douglas Stevenson, head of the Center for Seafarers' Rights of Seamen's Church Institute (SCI) in New York.

"They don't understand why they're being treated like terrorists, why they're confined to their ships, in some cases under guard," Stevenson said.