Nine radical Islamists jailed for their links to the 2003 Casablanca suicide bombings that killed 45 people broke out of a Moroccan top security jail on Monday, the Justice Ministry said.

The nine men escaped from Kenitra prison, 40 km (25 miles) east of Rabat, the ministry said.

"All the measures have been taken to arrest the escaped detainees," it added in a statement, saying authorities were investigating how the prisoners broke free.

A security source told Reuters the prisoners escaped through a tunnel they dug out under their cells.

"They literally saw the light at the end of the tunnel in the early hours today," the source said.

The escaped prisoners left a five-line letter, saying their breakout was the only solution to what they called injustice.

Islamist prisoner rights advocacy group Ennassir said the jailbreak coincided with the beginning of a one-day hunger strike by about 1,000 Islamist prisoners held at several prisons across Morocco, including Kenitra's.

"Most of the escaped prisoners had been sentenced to life imprisonment late in 2003 for their links with 2003's Casablanca bombings. It is the first such jailbreak," Ennassir chairman Abderrahim Mohtad told Reuters.

Mohtad said the Islamist prisoners were fasting on Monday to protest against what they called mistreatment and repression by prison officials. Authorities were not immediately available to comment on the prisoner allegations.

The prisoners said in the short letter, of which Reuters obtained a copy: "No to injustice. We had tried every way to end this injustice and we knocked on all doors for that without result. The only way left for us is to do that (breakout). We hope that you will understand.

"We apologise for the disturbance we have caused. That was the only solution," added the letter written in tentative Arabic and with the name of the nine prisoners. Morocco's prisons are overcrowded and squalid and most of the 60,000 prisoners complain of lack of decent food and access to healthcare, human rights groups say.

Islamist detainees want to be given "political prisoner status" which would allow them better conditions.