The White House and other major institutions in the US and South Korea could be hit by a fresh wave of cyber attacks, according to web security experts. The attacks last weekend slowed down computers in several institutions including the Pentagon, the US National Security Agency and the New York Stock Exchange.“This one was unusual in as much as it was so remarkably successful,” said Ted Bridis, an expert in internet technology. “Normally, even the most powerful denial-of-service attacks is mitigated after a few hours, certainly within a day. This one went on for several days.” South Korea’s spy agency has suggested that North Korea or its sympathisers may be behind the attacks. The North has warned of “high-tech war” against the South for spreading “false information”. But the US government and internet experts said it was too soon to make such claims.