North Korea has sharply criticized the U.S. after the U.S. Pacific Command moved a set of warships to the Korean Peninsula early April.
North Korea's foreign ministry, in a statement carried by its KCNA news agency on April 11, said the U.S. 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 United States."
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 United States.
Hwang Pyong-so, North Korea'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 April 8, the U.S. 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 U.S. response to North Korea's recent missile launches.
On April 5, 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.
Speaking to Al Jazeera from Seoul, BJ 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 program 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 United States.
U.S.-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 North Korea's activities and to ensure close communication with the United States.
"It is possible North Korea 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.
North Korea convened a Supreme People's Assembly session on April 11, one of its twice-yearly sessions in which major appointments are announced and national policy goals are formally approved.
April 15 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 North Korea'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 U.S. has been responding to it," he said.
"Seoul is waiting for words of reconciliation or at least expressions of interest in dialogue from Pyongyang."