South Korean police said Wednesday they have strengthened security against possible terror attacks by North Korea, amid high tensions during this month's US-South Korean military exercises.

The North's military calls the drills a rehearsal for invasion and threatens to turn Seoul into a "sea of flames" in case of any provocation.

Police SWAT team and sniffer dogs are patrolling major subway stations and airports and security at other major public facilities has also been tightened, the National Police Agency said in a statement.

Police commandos and armoured vehicles have been deployed to central Seoul to protect the US embassy, it said.

Pyongyang has labelled the regular Key Resolve/Foal Eagle drills, involving some 200,000 South Korean and 12,300 US troops, as an act of aggression and threatened retaliation.

Since the drills began Monday, the South's police have conducted training against "various forms of unconventional warfare and infiltration by enemy special forces", the statement said.

The joint exercises reportedly include scenarios such as countering localised provocations, tracking weapons of mass destruction, a sudden regime change in the communist state and an exodus of refugees.

Key Resolve, a command post exercise involving computer simulations, will last until March 10. Parts of Foal Eagle, a joint air, ground and naval training exercise, will continue through April 30.

The drills follow a year of high tensions.

The South accuses the North of torpedoing a warship in March 2010 near the disputed Yellow Sea border with the loss of 46 lives, a charge it denies.

Last November the North shelled a South Korean island near the border, killing two marines and two civilians.

The South's military will soon deploy light attack 500MD helicopters on frontline islands to counter any hovercraft infiltration by the North's special forces, Yonhap news agency quoted an unidentified military official as saying.