(CNN) -- Six UK soldiers are missing and presumed dead following an explosion in southwest Afghanistan, the Defense Ministry said Wednesday.
The incident occurred Tuesday in Helmand province.
"I have the tragic duty to report that six soldiers are missing, believed killed, during a security patrol," said Lt. Col. Gordon Mackenzie, a spokesman for a task force of soldiers in the province.
The soldiers were traveling in a tracked armored vehicle when the blast occurred, a British military official said. The vehicle hit a land mine, setting off a substantial explosion, the official said.
The area where the blast occurred was not on a paved road. It is in the Nar-e-Saraj area of Helmand, where Britain has lost many troops previously.
"No armoured vehicle ever developed will provide the occupants with 100% protection against all potential threats, including IEDs (improvised explosive devices)," said Chris Foss, a defense specialist at IHS Jane's, a think tank specializing in global security issues.
He said IEDs are "by far the biggest threat encountered" by international and national Afghan forces, though he said "significant progress has been made in neutralising the network and countering the IED threat."
There is a possibility that Tuesday's blast was caused by a "legacy device," a mine left over from the Soviet era, the military official said. Such devices have been found in that area, but there is no evidence leading to that conclusion at this point, he added.
It is also possible, even if it was an old mine, that it had been replanted by insurgents, he said.
Members of the NATO-led International Security Assistance Force reached the area, but will not officially confirm the deaths until they get into the damaged vehicle.
If confirmed, the deaths in Tuesday's attack would be the most British troops killed in a single day in Afghanistan since six were killed on July 10, 2009, according to a CNN tally.
UK officials decried the attack.
"I utterly condemn those responsible for this incident, who will ultimately fail to derail a mission that is protecting our national security at home and making real progress in Helmand Province, a testament to the bravery, commitment and professionalism of our armed forces," said Defense Secretary Philip Hammond.