Singapore commissioned two new advanced submarines on Tuesday, vessels its navy says are meant to protect sea lines of communication and which experts said would range beyond the waters of the tiny city-state to keep tabs on the region.

The country's fifth and sixth submarines, the Invincible and the Impeccable, were ordered from Germany's ThyssenKrupp Marine Systems in 2013 in a €1 billion ($1.11 billion) deal, and join older Archer— and Challenger-class boats.

They represent a technological step forward for Singapore's fleet, experts say, allowing the wealthy Southeast Asian nation to operate farther from shore using fewer sailors, and making its craft among the quietest in the region.