The mystery of 50 missing North Korean submarines deepened fears yesterday over Pyongyang’s threat to fire across the border.

South Korea yesterday said the submarines, comprising about 70 per cent of a North Korean submarine fleet, had left their home ports and were nowhere to be found.

US and South Korean reconnaissance planes and naval vessels searched yesterday off the east and west coasts of the Korean peninsula for the submarines. Military officials said deployment of the submarines was the largest in the region since the Korean War.

The question is whether the 50 submarines, 1,400-ton Romeo-class and 1,000-ton Whiskey-class vessels, have orders to target commercial or naval vessels – or are just putting on a show of force. “No one knows,” said a military spokesman. “We are mobilising all our surveillance resources.”

Concern about the missing submarines compounded worries as North Korea doubled the amount of heavy artillery and other weapons massed on its side of the Demilitarised Zone (DMZ) that has divided the two Koreas since the truce ending the Korean War was signed in 1953.

The artillery equipment that has been moved forward will be able to fire accurately on the banks of huge loudspeakers on the South Korean side of the ﻿4km-wide DMZ, which have been broadcasting news and music loud enough to be heard by North Korean troops at least 10 miles away.