Seoul (AFP) - A US citizen who said he had crossed illegally into North Korea held a press conference in Pyongyang Sunday to denounce his own country's US political and economic systems.

The man identified himself as Arturo Pierre Martinez, aged 29 and from El Paso in Texas, CNN and the North's official KCNA news agency reported.

He entered the North from China in November, according to a North Korean statement cited by the broadcaster, just two days after the US spy chief James Clapper arrived in Pyongyang to secure the release of two other detained Americans.

Martinez had earlier tried to swim across the Han river between the two Koreas to enter the North, only to be caught and sent back to the US where he was placed in a psychiatric hospital, his mother told CNN.

"But he got out," Patricia Eugenia Martinez said, adding her son was bipolar.

"He...got the court to let him out and instead of coming home to us he bought a ticket and left for China," she said.

At the press conference Martinez admitted the illegal entry but said he was "extremely grateful" for the North to have pardoned him and to have offered him "the most generous reception", according to KCNA.

Images obtained from the North's government showed him wearing a suit and tie and delivering a statement.

The North aired the criticisms by Martinez of his own government at a time when Pyongyang is under growing international pressure to improve its human rights record.

The UN Security Council is expected to meet this month on the North's rights record, following a proposal to refer it to the International Criminal Court on possible charges of crimes against humanity.

A US State Department official said it was aware of reports that an American citizen had crossed into North Korea.

"The Department of State strongly recommends against all travel by US citizens to North Korea," the official said.

"We have no additional information to share at this time."

Martinez said he plans to seek political asylum in Venezuela, another ideological foe of the United States.

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He added that he had been staying at a "very nice hotel" while being investigated for illegal entry.

South Korean troops in September caught a US man trying to swim across the river border with the North but did not disclose his identity.

The man, reported to be in his late 20s or early 30s, told Seoul authorities that he had wanted to meet the North's leader Kim Jong-Un, Yonhap news agency reported at the time.

KCNA said Martinez was "staying" in North Korea. It was not known whether or when he would be able to leave the country, which has a history of detaining US citizens.

The North in October freed 56-year-old US national Jeffrey Fowle, who was detained after allegedly leaving a Bible in a nightclub bathroom.

The following month it freed US missionary Kenneth Bae and another American, 24-year-old Todd Miller, following the secret mission by Clapper.

Bae was arrested in November 2012 and later sentenced to 15 years' hard labour after being accused of plotting to overthrow the isolated state's regime.

Miller was arrested in April after allegedly ripping up his visa at immigration. He was sentenced to six years' hard labour in September.

At Sunday's press conference Martinez accused the US government of operating like "a Mafia enterprise" plundering other nations.

"The democracy of this nation is an illusion and its representatives act as nothing more than powerbrokers for those who can offer them," he was quoted by KCNA as saying.