North Korea accused South Korea's military of opening fire Sunday towards its side of the tense land border, in what it termed a "grave armed provocation". It said the South's military fired 90 mm recoilless guns towards a civil police post in the North on the eastern sector of the border early afternoon. The firing, "seriously threatening the safety of civil policemen of the north side on routine duty", was designed "to deliberately aggravate the situation" in the border buffer zone, the official Korean Central News Agency said. A "touch-and-go situation" prevails along the border due to such undisguised military provocation, it said. Seoul's forces have been ordered on heightened alert since the mysterious sinking of a South Korean warship near the disputed sea border with the North on March 26. South Korea has not so far accused the North of involvement in the sinking, while the North has remained silent about the incident which left 46 sailors missing and feared dead.