A series of memorials are underway in the United States as the country remembers those killed in the 9/11 terrorist attacks.

Events are taking place at the four sites struck by hijacked planes in New York, Washington and Pennsylvania.

Security across the country has been tightened following warnings of possible new attacks.

The CIA has warned it has received “credible but unconfirmed” threats.

Nearly 3,000 people were killed during the attacks, which destroyed the World Trade Centre in New York and parts of the Pentagon in Washington.

President Barack Obama has said the US remains vigilant against terrorism.

There will be pauses for silence at the exact times when two airliners crashed into the twin towers of the World Trade Centre in New York.

A third plane was crashed into the Pentagon while a fourth was forced into a field in Shanksville, Pennsylvania, by passengers who fought with the hijackers.

A new National September 11th Memorial, featuring two reflecting pools, each almost an acre in size, in the footprints of twin towers, will be unveiled.

The names of those who died on 9/11, as well as the six people killed in the 1993 World Trade Center bombing, are inscribed on the edge of the pools.