More than 300,000 people have been displaced by flooding in Malawi, almost twice as many as previously estimated, the United Nations Children’s Fund, or Unicef, said Thursday. Floods set off by weeks of heavy rains have affected more than one million people, killed 276 and injured more than 600, according to new figures released by the United Nations Disaster Assessment and Coordination unit. Unicef said that its staff members were on high alert for waterborne diseases such as cholera, dysentery and malaria and that displacement camps were providing about 56,000 women and children with essential health care services.