North Korea said Tuesday it possessed rockets capable of striking the US mainland, as it slammed a new US-South Korean deal to extend the range of the South's missile systems as a provocation for war.

The threat, which analysts largely dismissed as bluster, came after South Korea announced Sunday an agreement with the US to almost triple the range of its missiles to 800 kilometers to cover the whole of North Korea.

A spokesman for the North's National Defense Commission said the deal was "another conspiracy of the master and the stooge to push the situation on the Korean Peninsula to the extreme... and ignite a war."

In a statement carried by the official Korean Central News Agency, he said the North's military, including "strategic rocket forces," had a "scope of strike" that not only covered US and South Korean bases in South Korea, "but also Japan, Guam and the US mainland."

UN Secretary General Ban Ki-moon on Tuesday said North Korea's claim is "alarming," .

Ban, a former foreign minister of South Korea, admitted he had no way of knowing if the claim was credible but said, "I read the report and it is quite an alarming statement by North Korea."

However, South Korean analysts said the claim was likely to be empty rhetoric, suggesting it was aimed more at boosting military morale under new leader Kim Jong-un.

"There is no evidence that North Korea has succeeded in tests of a missile with a range long enough to hit the US mainland," said Yun Duk-min, a professor at Korea National Diplomatic Academy.

North Korea is known to have an inter-continental ballistic missile (ICBM) in development, the Taepodong-2, but it has never been tested successfully.

In April, North Korea failed with a much-hyped rocket launch that Pyongyang said was aimed at placing a satellite in orbit.

The US and United Nations insisted it was a disguised ballistic missile test using a three-stage variant of the Taepodong-2.

Days after the failed launch, North Korea raised eyebrows by displaying what appeared to be a new set of ICBMs at a military parade to mark the 100th birthday of the North's late founder Kim Il-sung.

But Western military analysts and UN sanctions experts concluded that the display models were simply mock-ups.

The new agreement announced Sunday with the US allows the South to deploy missiles with a range of 800 kilometers, up from the current limit of 300 kilometers.