Travellers are being warned to expect tighter security measures at airports and public events as the country’s intelligence agencies try to combat the threat of a so-called “lone wolf” terrorist attack.

The domestic spy agency, ASIO, yesterday lifted the nation’s terrorism alert level from medium to high — meaning an attack is considered likely.

However, Prime Minister Tony Abbott has stressed there is no suggestion of an imminent attack.

Security agencies are keeping watch on a number of people who are of interest, but it is the threat of someone acting alone that has them most concerned.

It means there will be extra security at airports, public events and government buildings.