The United States today announced new sanctions against North Korea, targeted against its leadership, and warned of serious consequences if it again attacked the South.Addressing to a news conference in Seoul with US Defense Secretary Robert Gates after a visit to the demilitarised zone dividing the peninsula,US Secretary of State Hillary Clinton said that Washington was ready to return to international talks over North Korea's nuclear weapons programme if Pyongyang sent a positive signal, but that there had been none so far.`Relations across the divided peninsula have turned increasingly hostile after South Korea accused the North of sinking one of its warships in March, killings 46 sailors.The sanctions announced by the US will not really affect the North Korean economy as all possible sanctions have already been imposed.The South Korean and US foreign and defence ministers warned of serious consequences'' if there were any future North Korean attacks against the South.