The US State Department has admitted it is offering a reward of up to $10m for information leading to the arrest of anyone involved in last year's attack on a US diplomatic compound in Libya.

In a letter sent to US politician on Friday, the Department said the rewards were not advertised on its "Rewards for Justice" website, as is normal, due to security issues around the ongoing investigation.

The US ambassador to Libya, Chris Stevens, was one of four Americans killed in an attack on the US mission in Benghazi on September 11, 2012.

A State Department official revealed that a letter to Republican Michael McCaul acknowledged that it was not unusual not to publicize offers of rewards, and that they had ways of making sure the information was known "as needed".

The official added that the rewards had been in place since January 7. 

US politicians had complained that the department was not using all the tools at its disposal to catch those who carried out the attack.

The State Department official said the rewards were not targeted at specific individuals but were intended to solicit information about anyone who took part in the attack.