South Korea has begun annual military exercises with the US and says the North has apparently cut off a hotline used to maintain the armistice that Pyongyang has recently repudiated over nuclear test sanctions.

After the start of the drills, South Korean officials said their northern counterparts didn't answer two calls on the hotline between the sides, apparently following through on an earlier vow to end non-aggression measures.

Pyongyang has launched a propaganda campaign against the drills, which involve 10,000 South Korean and about 3,000 American troops, and last week's UN vote to impose new sanctions over the North's 12 February nuclear test.

Pyongyang isn't believed to be able to build a warhead small enough to mount on a long-range missile and the North's military has repeatedly vowed in the past to scrap the 1953 armistice. North Korea wants a formal peace treaty, security guarantees and other concessions as well as the removal of 28,500 US troops stationed in South Korea.

North Korea regularly claims South Korea-US drills are a preparation for invasion but Pyongyang has shown heightened anger over the exercises that began on Monday. Before the UN vote it threatened to fire a nuclear missile at the US and has also warned South Korea of a nuclear war on the divided peninsula.

Under newly inaugurated President Park Geun-hye, South Korea's defence ministry, which often brushes off North Korean threats, said the North's government would "evaporate from the face of the Earth" if it ever used a nuclear weapon. The White House also declared the US was fully capable of defending itself against a North Korean ballistic attack.

North Korea has said the US mainland is within the range of its long-range missiles and an army general has told a Pyongyang rally that the military is ready to fire a long-range nuclear-armed missile to turn Washington into a "sea of fire".

But there are still worries about a smaller conflict. North Korea has a variety of missiles and other weapons capable of striking South Korea. In 2010 North Korea shelled a South Korean island and allegedly torpedoed a South Korean warship, killing a total of 50 South Koreans. Both incidents occurred near the disputed western sea boundary, a recurring flashpoint between the Koreas that has seen three other bloody naval skirmishes since 1999.

Kim Jong-un visited two islands just north of the sea boundary last week and ordered troops there to open fire immediately if a single enemy shell was fired on North Korean waters.

The US and the South are holding 11 days of drills as part of two months of war games.