South Korea's acting president warned on Tuesday of "greater provocations" by North Korea as tension on the Korean peninsula rises over concern the North may conduct a test of its military hardware in coming days.

A U.S. Navy strike group led by a nuclear-powered aircraft carrier is en route to the western Pacific with talk of military action by the United States gaining traction following its strikes last week against Syria.

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

"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 former President Park Geun-hye was removed over a graft scandal.

The North convenes 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.

The North's foreign ministry, in a statement carried by its KCNA news agency earlier on Tuesday, said the U.S. navy strike group's move near the Korean peninsula 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," an unidentified ministry spokesman said.

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Delegates from around the North have been arriving in Pyongyang ahead of the assembly session. They visited statues of previous leaders Kim Il Sung and his son, Kim Jong Il, state media reported.

North Korea is emerging as one of the most pressing foreign policy problems facing the administration of U.S. President Donald Trump.

North Korea has conducted five nuclear tests, two of them last year, and is working to develop nuclear-tipped missiles that can reach the United States.

The Trump administration is reviewing its policy towards North Korea and has said all options are on the table, including military strikes.

The U.S. Navy strike group Carl Vinson cancelled a planned trip to Australia and was moving toward the western Pacific Ocean near the Korean peninsula as a show of force, a U.S. official told Reuters over the weekend.

Trump and his Chinese counterpart, Xi Jinping, met in Florida last week and Trump pressed Xi to do more to curb North Korea's nuclear programme.

China and South Korea agreed on Monday to impose tougher sanctions on North Korea if it carried out nuclear or long-range missile tests, a senior official in Seoul said.

As well as the anniversary of Kim Il Sung's birth, there are several other North Korean anniversaries in April that could be opportunities for weapon tests, South Korean officials have said.

The North is seen ready to conduct its sixth nuclear test at any time, with movements detected by satellites at its Punggye-ri nuclear test site.