Pyongyang: North Korea has tightened border controls after a group of refugees defected to the South across the tense Yellow Sea border, an aid group said on Friday.
Nine North Koreans -- three men, two women and four children -- made an overnight journey in two small boats across the sea border on June 11 and said they wished to defect.
Seoul rejected the North`s demand that they be returned, triggering bitter protests from Pyongyang.
The defections prompted North Korea to ban small motorless boats along its west coast and to tightly screen people who want to go to sea, Good Friends, a Seoul-based welfare group with contacts in the North, said in its newsletter.
The North has also restricted travel to border areas, it said.
Some 21,000 North Koreans have fled to the capitalist South since the end of the 1950-1953 war, the vast majority in recent years, to escape repression or poverty and food shortages.
In February, a small boat packed with 31 North Koreans drifted to South Korea across the Yellow Sea border in thick fog.
South Korea returned 27 but refused to hand over the other four, saying they had freely chosen to stay in the South. Pyongyang complained that the four had been pressured to stay.
Cross-border tensions have been high for well over a year, since the South accused the North of torpedoing a warship in March 2010, killing 46 sailors.
Pyongyang denied the charge but went on to shell a border island last November, killing four South Koreans.
The North has threatened an attack in protest at the use by some South Korean troops of photos of Pyongyang`s ruling family as rifle-range targets. The practice has been stopped but the North is demanding an apology.
This week the North`s military warned of "merciless" retaliation for anti-Pyongyang slogans displayed by South Korean frontline troops.