The terror alert system brought in by the US government after the 9/11 attacks is to be scrapped and replaced with warnings on Twitter and Facebook.

Under a new system set to be introduced on 27 April, US terror alerts will have just two levels – elevated and imminent – and sometimes be broadcast on social networking websites.

The colour-coded warnings introduced almost 10 years ago will be phased out in the move – a big departure for security officials who have struggled with how much information to reveal about terror plots.

A 19-page Homeland Security document outlining the plans was obtained by the Associated Press. It describes the behind-the-scenes process when the US comes under attack.

Terror alerts may be broadcast on Twitter and Facebook only "when appropriate", according to the hitherto top-secret document. Terror threats are made public only when members of Congress, various counter-terrorism officials, governors and mayors have been informed. The document even specifies how many minutes US officials can wait before organising urgent conference calls to discuss pending threats.

The new advisory system is designed to be easier to understand and more specific. The present warning levels have been lampooned by comedians and criticised by TV broadcasters for being too vague and open to interpretation.

According to the draft plan, the Homeland Security unit will issue targeted warnings based on the threat. For example, if there is a specific threat that terrorists are looking to hide explosives in US airports, the government might issue a public warning that would be announced in airports telling travellers to be extra vigilant and report any suspicious activity.

Under the new system, an "elevated" alert would warn of a credible threat against the US. It would likely not specify timing nor targets, but it could reveal terrorist trends that intelligence officials believe should be shared in order to prevent an attack.

An "imminent" alert would warn about a credible, specific and impending terrorist threat or an ongoing attack against the US.