North Korea announced Thursday it has detained an American man it says illegally entered the country from China, the second U.S. citizen it has reportedly arrested in the past month.

The man was detained Monday and is under investigation, the North's official Korean Central News Agency said in a brief notice. It provided no other details.

A South Korean marine at Incheon port, west of Seoul, South Korea, watches a TV report about North Korea's firing of artillery near the disputed sea border. (Yonhap, Ha Sa-hun/Associated Press)

The announcement comes after North Korea fired artillery near its disputed western sea border with South Korea for a second straight day, heightening security concerns in the region.

South Korea had fired warning shots from a marine base on an island near the disputed sea border on Wednesday after the North fired about 30 rounds of artillery from its coast, but did not respond to the North's artillery fire on Thursday.

An official at Seoul's Joint Chiefs of Staff, speaking on condition of anonymity, said the shells the North fired on Thursday are believed to have landed in North Korean waters.

The missile tests were preceded by North Korea's designation of no-sail zones in disputed waters, through March 29. The two countries, technically still at war since 1950-53 Korean War ended with an armistice and not a peace treaty, have clashed in the past in waters claimed by both sides.

No information on detained American

U.S. State Department spokesman P.J. Crowley criticized the North for raising tension, saying the no-sail zone designation and the firing were "provocative actions and, as such, are not helpful."

But the State Department had no immediate comment on the report concerning the detained American. The U.S. Embassy in Beijing also said it had no information and the embassy in Seoul had no comment.

North Korea had said earlier in the month it was holding another U.S. citizen for illegally entering through the North Korea-China border.

North Korea did not identify the man in that instance either, but it is widely believed to be American missionary Robert Park, who South Korean activists say crossed over a frozen river into North Korea several days before to raise human rights issues.

U.S. officials are still seeking consular access to that citizen, Crowley said Wednesday.

Diplomatic negotiations stall with North Korea

The naval tensions between North and South Korea and the detainment of the U.S. citizen come as South Korea and regional powers continue complicated diplomatic negotiations over the North's nuclear weapons program.

In recent weeks North Korea has alternated between overtures of friendship and warnings of retaliation with its southern neighbour.

Multilateral talks between the North and regional powers — including the U.S., China and Japan — have also stalled because of the North's insistence on one-on-one talks with the United States to formally end the Korean War.

With files from The Associated Press