No explanation was given officially, but an informed Yemeni source said the order was the latest move in Yemini government crackdown on Islamic fundamentalists following the killing of three U.S. Baptist missionaries earlier this week, and -- in a separate action -- the assassination of the leftist leader of the oppposition.
In recent months, there has been a sharp increase in the number of military personnel who have grown beards as a sign that the wearer is a Muslim fundamentalist, or a sympathizer, the source said.
Friday's order said that any soldier reporting to his barracks still wearing a beard would not be allowed to report for duty.
"I was forbidden from entering my barracks near Sanaa International Airport until I shaved my beard," one soldier told United Press International. "Such measure didn't exist before."
Earlier this week Yemini authorities arrested 13 men, described as Islamist militants, in connection with the killing of three U.S. missionaries from a Baptist mission hospital in Jibla, southern Yemen, and wounding a fourth.
All 13 were said to have ties with Abed al-Kamel, already held on suspicion of Monday's shooting of the four Americans, and Ali Jarallah, held on suspicion of assassinating leftist opposition leader Jarallah Omar, deputy head of the Yemini Socialist Party.
Yemini police said they believe that both Jarallah and al-Kamel are linked to Osama bin Laden's al-Qaida terrorist network, which has camps and training bases in Yemen.
Scores of other known or suspected Islamist militants were also arrested. They included mullahs who have denounced Yemen's coopertion in the U.S.-led war on terrorism at Friday prayers.
Initial investigation indicated that the two suspects planned the two attacks together, and a third accomplice was supposed to carry out an attack against a residential compound inhabited by students from Yemen's Bahari community, a sect connected with the Shitte Muslims.
Investigators also revealed that Ali Jarallah had planned a series of attacks against secular party leaders and journalists.
But other Yemeni Muslim clerics condemned Omar's assassination and the killing of the U.S. missionaries.