South Korea has dispatched its anti-piracy naval unit to waters off Somalia after communication with a Mongolian fishing vessel was cut when it requested help, fearing it was being followed by a suspected pirate vessel.
South Korean foreign ministry said communication with the squid fishing vessel, with three Koreans and 18 Indonesians on board, was cut after midnight on Friday.
The ministry said India, Germany and Japan have deployed aircraft to the waters off Somalia to search for the vessel.
"We are working with authorities in related countries including the United States, Germany, India and Japan," the ministry said in a statement.
It said South Korean President Moon Jae-in asked related ministries to "put the utmost priority on human lives and make sure that they will be rescued".
A South Korean navy unit in 2011 stormed a South Korean-operated chemical carrier Samho Jewelry that had been hijacked by Somali pirates, rescuing all 21 crew members and killing eight pirates.
On Tuesday, Somali pirates hijacked an Iranian fishing vessel to use as a base to attack bigger, more valuable ships, part of an upsurge in attacks following years of relative calm.
This month has seen a new rash of attacks, with two ships captured and a third rescued by Indian and Chinese forces after the crew radioed for help and locked themselves in a safe room.
The sudden string of attacks by Somali pirates comes after years without a reported incident. Attacks peaked with 237 in 2011 but declined steeply after ship owners improved security measures and international naval forces stepped up patrols.
The United States is closely watching a recent increase in piracy off the coast of Somalia and a senior U.S. military official says the rise in piracy attacks has at least partially been driven by famine and drought in the region.