North Korea has sharply criticised the US after the US Pacific Command moved a set of warships to the Korean Peninsula over the weekend.

The North's foreign ministry, in a statement carried by its KCNA news agency on Tuesday, said the US navy strike group's deployment showed America's "reckless moves for invading had reached a serious phase".

"We never beg for peace but we will take the toughest counteraction against the provocateurs in order to defend ourselves by powerful force of arms and keep to the road chosen by ourselves," a spokesman for the country's foreign affairs ministry said.

"The DPRK [North Korea] is ready to react to any mode of war desired by the US."

Later in the day, North Korea's military chief said his country was ready to "mount a pre-emptive nuclear attack" on South Korea and the US.

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Hwang Pyong-so, the North's effective number two behind leader Kim Jong-un, made the threat during a live broadcast on state television.

He insisted North Korea will "wipe them out without a trace if they attempt to launch a war of aggression".

On Saturday, the US warships - including the aircraft carrier Carl Vinson, two guided-missile destroyers and a guided-missile cruiser - cancelled a trip to Australia and headed from Singapore to the waters off Korea, as part of the US response to North Korea's recent missile launches.

On Wednesday North Korea launched a missile into the Sea of Japan from near Sinpo in South Hamgyong province, according to South Korea's joint chiefs of staff.

Tensions rising

Speaking to Al Jazeera from Seoul, B J Kim, adjunct professor at Hankuk University of Foreign Studies, said the level of tension has many South Koreans worried.

"The overall situation here, the way the South Koreans perceive it, is very unusual. They have not seen this level of heightened tensions for about a quarter of a century," Kim said.

"In 1994 we had a similar situation in which the United States possibly wanted to strike. But since then this has been the highest point of tensions here, so people feel quite uneasy about it."

North Korea has ratcheted up its nuclear programme under its relatively new leader Kim Jong-un, carrying out two nuclear tests and launching around 20 ballistic missiles last year alone.

The international community also is concerned that North Korea could be working on an intercontinental ballistic missile, which could reach the western US.

US-based experts say that North Korea is currently planning a further nuclear test.

Hwang Kyo-ahn, South Korean acting president, ordered the military to intensify monitoring of the North's activities and to ensure close communication with the ally the US.

"It is possible the North may wage greater provocations such as a nuclear test timed with various anniversaries including the Supreme People's Assembly," said Hwang, acting leader since Park Geun-hye was removed as president over a corruption scandal.

Important date

The North convened a Supreme People's Assembly session on Tuesday, one of its twice-yearly sessions in which major appointments are announced and national policy goals are formally approved.

Saturday is the 105th anniversary of the birth of Kim Il-sung, the country's founding father and grandfather of current ruler, Kim Jong-un.

A military parade is expected in the North's capital, Pyongyang, to mark the day.

North Korea often also marks important anniversaries with tests of its nuclear or missile capabilities.

Hankuk University's Kim said South Korea feels it is up to North Korea to open the possibility of dialogue.

"North Korea has been escalating the tensions and the US has been responding to it," he said.

"Seoul is waiting for words of reconciliation or at least expressions of interest in dialogue from Pyongyang.

"From a Chinese perspective, they have always stressed the need for dialogue but we have not seen any progress for years now. China is therefore also responsible for coming up with new ideas to lower tensions."