WASHINGTON (Reuters) - U.S. President Donald Trump ordered his homeland security adviser, Tom Bossert, to hold an emergency meeting Friday night to assess the threat posed by a global computer ransomware attack in at least 150 countries, a senior administration official told Reuters. Senior security staff held another meeting in the White House Situation Room on Saturday, and the FBI and National Security Agency were trying to identify the perpetrators of the massive cyber attack, said the official, who spoke on condition on anonymity to discuss internal deliberations. 