A three-year prison sentence handed to a blogger who criticized Egypt's army suggests the country's military rulers are drawing red lines around permissible speech, Human Rights Watch said.

The military council ruling Egypt said 25-year-old activist Maikel Nabil had used inappropriate language and defamed the military, and that his call that military conscription be scrapped would have a negative effect on young Egyptians.

Army officers arrested Nabil on March 28 at his home in Cairo and the military prosecutor charged him with insulting the military establishment and spreading false information, said New York-based Human Rights Watch (HRW).

Nabil's lawyers were told the judge would rule on April 12 but they discovered on April 11 that he had already been sentenced a day earlier in their absence, HRW cited defense lawyer Adel Ramadan as saying.

Maikel Nabil's three-year sentence may be the worst strike against free expression in Egypt since the Mubarak government jailed the first blogger for four years in 2007, said Joe Stork, deputy Middle East director at Human Rights Watch.

The sentence will only be final once ratified by the chief of the military district, said HRW, calling on the army to drop all charges against Nabil and release him immediately.

Since a popular uprising ousted President Hosni Mubarak on February 11, the country's military rulers have promised free and fair elections and an end to three decades of emergency rule.

Heads of state media seen as too close to Mubarak's government have been replaced and a lively public debate on issues ranging from high-level corruption to government mismanagement is under way.

The army enjoys broad popular support, but has come under growing criticism from pro-democracy activists who accuse it of unfairly detaining peaceful protesters.

HRW said it had seen a letter sent to Egyptian newspaper publishers from the ruling military council which said any news, pictures or press releases concerning the armed forces must be vetted first by its Morale Affairs directorate and military intelligence in order to protect national security.

State institutions, including the military, should never consider themselves above criticism, said HRW's deputy Middle East director Joe Stork. It is only through a public airing of abuses and full accountability measures that Egypt can hope to transition away from past human rights violations.

(Reporting by Tom Pfeiffer; Editing by Jon Hemming)