Image caption Spies were told to watch out for secret messages written on body parts and engraved on toe-nails.

World War I spies engraved messages on toe-nails and used lemon juice to write invisible letters, classified documents released by the CIA reveal.

The six documents, amongst the oldest secret papers to be held by the agency, disclose a number of spying techniques.

The nearly century-old records include instructions "to suspect and examine every possible thing".

Recent advancements in technology have made it possible to release the documents, the CIA said.

One document suggests soaking a handkerchief, or any other starched substance, in nitrate, soda and starch, in order to make a portable invisible ink solution.

Putting the treated handkerchief in water would release a solution that could then be used to write secret messages, the records say.

A document written in 1914 in French, exposes a German formula for making secret ink, suggesting that French spies had managed to crack the enemy's code.

'Spies and smugglers'

One memorandum, compiled by a hand-writing expert in California, suggests painting invisible messages on the human body.

"To make them appear, develop a suitable reagent sprayed with an atomizer" the record states.

The document warns of "other methods used by spies and smugglers, according to the skill and education of the criminals", such as "engraving messages and credentials on toe-nails".

The secrets have been made obsolete by advances in the chemistry of secret ink and the lighting methods used to detect it , the CIA said.

The CIA declassified more than a million historical documents last year. They are available on the agency's website.