South Korean and US forces are taking part in military exercises amid tensions over North Korea's missile and nuclear programmes.

South Korean President Moon Jae-in said the joint drills, called Ulchi Freedom Guardian, were purely defensive and did not aim to raise tensions on the peninsula.

"There is no intent at all to heighten military tension on the Korean peninsula as these drills are held annually and are of a defensive nature," Mr Moon told Cabinet ministers.

"North Korea should not exaggerate our efforts to keep peace nor should they engage in provocations that would worsen the situation, using [the exercise] as an excuse," he said.

The joint annual US-South Korean drills will continue to 31 August and involve computer simulations designed to prepare for war with a nuclear-capable North Korea.

The US also describes them as "defensive in nature", a term North Korean state media has dismissed as a "deceptive mask".

"It's to prepare if something big were to occur and we needed to protect ROK," said Michelle Thomas, a US military spokeswoman, referring to South Korea by its official name, the Republic of Korea.

North Korea views such exercises as preparations for invasion and has fired missiles and taken other actions to coincide with the military drills in the past.

North and South Korea are technically still at war after the 1950-53 Korean War ended with a truce, not a peace treaty.

North Korea's rapid progress in developing nuclear weapons and missiles capable of reaching the US mainland has fuelled a surge in regional tension and UN-led sanctions appear to have failed to bite deeply enough to change North Korea's mind.

US President Donald Trump has warned that North Korea will face "fire and fury" if it threatens the United States.

The North responded by threatening to fire missiles towards the US Pacific island territory of Guam, but later said it was holding off on a decision while it waited to see what the United States would do next.

China has renewed its call for the US and South Korea to suspend the joint military drills.

"The current situation on the Korean peninsula is highly sensitive and fragile, which requires the directly concerned parties including South Korea and US to make joint efforts to ease the tensions," Chinese foreign ministry spokeswoman Hua Chunying said.

Beijing has pleaded for a resumption of long-dormant six-nation talks to peacefully resolve the nuclear crisis.

But its proposal for North Korea to suspend its arms programmes in return for the US to halt military drills in the region has been ignored.

"We don't think the joint exercise will be conducive to easing the current tensions. And we urge relevant parties to (seriously consider) the suspension-for-suspension proposal," Ms Hua told a regular press briefing.