SEOUL (AFP) - North Korea pressed ahead with a live-fire artillery drill in a tense border zone on Thursday despite US and South Korean criticism, and separately announced it had detained an American citizen. The communist state fired shells into the sea near its disputed maritime border with South Korea for a second day, Seouls military said. Pyongyangs official news agency said an American was under investigation after being detained Monday for illegally crossing the border from China the second time in a month it has reported such a case. The shells from shore batteries landed near a South Korean island but Seoul said none fell in S.Korean waters. On Wednesday the North had lobbed more than 80 shells near the flashpoint frontier and South Korean Marines fired 100 warning shots in response. The North describes the drill, which started a day after it declared no sail zones in the area, as a routine military exercise. But the western sea border, which it refuses to recognise, was the scene of deadly naval battles in 1999 and 2002. In the latest naval clash last November, a firefight left a North Korean patrol boat in flames. Washington called the shelling provocative, echoing an earlier protest from Seoul. The declaration by North Korea of a no sail zone and the live firing of artillery are provocative actions and as such are not helpful, State Department spokesman Philip Crowley said. Analysts said the North is raising tensions to show that a formal peace pact on the heavily-armed peninsula is necessary. Before it returns to nuclear disarmament negotiations, Pyongyang wants Washington to agree to discuss a treaty to replace the 1950-53 Korean War armistice. But the latest reported detention is a potential complication in Pyongyangs stated desire to improve relations. The North did not identify the man or give any details. The US State Department and South Koreas foreign ministry said they knew nothing about the case. It was unclear if there was any connection to US missionary Robert Park, who was picked up on December 25 as he crossed the border from China on a lone human rights crusade. Jo Sung-Rae, a South Korean Christian involved in Parks case, told AFP: Right now we have no information about the reported entry of another American. Park, 28, claimed he had seen a vision from God of North Koreas liberation and redemption, according to his colleagues. They said he carried a letter calling on North Korean leader Kim Jong-Il to release political prisoners, shut concentration camps and improve rights and conditions. After months of tensions marked by a nuclear test and missile launches, the North last summer began peace overtures to Seoul and Washington. Some analysts said the North hopes to ease tougher sanctions imposed following the weapons tests. US President Barack Obama sent envoy Stephen Bosworth to Pyongyang last month to try to persuade it to return to the six-nation nuclear talks. But the North says that in addition to peace treaty talks, it wants sanctions lifted before it comes back. Obama in his State of the Union speech Wednesday warned Pyongyang it faces growing isolation unless it abandons its nuclear weapons. North Korea now faces increased isolation, and stronger sanctions sanctions that are being vigorously enforced, he said. In recent weeks the North has sent mixed messages to Seoul. It is pressing to upgrade or restart joint business projects but its military has threatened possible attacks. The North was angered by media reports that the South has drawn up a contingency plan in case of regime collapse in Pyongyang. Seouls warning that it would launch a preemptive strike to foil any threatened nuclear attack also prompted an irate response. South Korean authorities are hatching plots to harm fellow countrymen at any cost, letting loose a string of extremely bellicose outbursts, blinded with the ambition to invade the North, said Rodong Sinmun, newspaper of the ruling communist party, on Thursday.