World leaders have extended their condolences to France over the terror attack at Champs Elysées that has left one police officer dead and two injured.

US President Donald Trump spoke candidly in response to the attack. At a news conference in Washington with visiting Italian Prime Minister Paolo Gentiloni Silveri, he said: "It looks like another terrorist attack. What can you say? It just never ends."

"We have to be strong and we have to be vigilant, and I have been saying it for a very long time."

US Vice-President Mike Pence said the Paris shooting is the "latest reminder that terrorism can strike anywhere, anytime."

He also said that the US "will not relent in our effort to end terrorism."

Presidential candidates Francois Fillon and far right Front National candidate Marine Le Pen have cut their campaigns short as a mark of respect. The televised presidential debate was interrupted to broadcast the news of the shooting.

French authorities have also warned their citizens to be vigilant as police continue to investigate whether the assailant, who was shot dead by police officers at the scene, had any accomplices.

French President Francois Hollande said: "A national tribute will be paid to this policeman who was killed in such a cowardly way."

"The fight against terrorism must be the absolute priority of the next French president," Fillon said.

Other heads of state have responded to the Paris attack. Australian Prime Minister Malcolm Turnbull offered his country's prayers to the police officers who were shot. He also urged Australian travellers to check for security warnings on the Australian Department of Foreign Affairs and Trade website.

"Everywhere - but especially in Europe at the moment - pay close attention to your surroundings."

A Downing Street spokesperson said: "The UK strongly condemns the appalling terrorist attack in Paris. The prime minister has tonight [20 April] passed on her condolences to President Hollande."

The UK Foreign Office has also issued a warning to all British nationals in Paris to "remain vigilant and follow the advice of the local security authorities."

In March, PC Keith Palmer and three others were killed when Khalid Masood collided into pedestrians with his car on Westminster Bridge in London before crashing into the gates of Parliament. He then got out of the car and fatally stabbed Palmer.

Steve White, the chairman of the Police Federation of England and Wales, tweeted: "As French colleagues felt the pain of our loss in Westminster last month, we feel theirs tonight."

France has been in a state of emergency since the 2015 Paris attacks which left 130 people dead. In December 2016, parliament voted to extend the state of emergency ahead of the upcoming presidential and general elections.