London, Aug. 19 : Russia has reportedly placed short range SS-21 missiles in South Ossetia, that could pose a threat to most major Georgian cities," including the capital, Tbilisi, a U. S. defense official confirmed to Fox News on Monday.

Pentagon spokesman Bryan Whitman was quoted as saying that: "The only forces that are permitted to remain under the cease fire agreement are the forces that were in there at the Aug. 6 time frame."

"Anything such as that, or any other military equipment that was moved in would be in violation of this cease-fire and should be removed immediately," he added.

This news came as Russia''s deputy chief of staff, Colonel General Anatoly Nogovitsyn, said that the withdrawal of Russian peacekeepers and reinforcements has begun" and added that the forces were leaving Gori.

The U. S. State Department, however, has been unable to confirm a Russian troop pullout.

According to the European Union-brokered peace plan signed by both Medvedev and Saakashvili, both sides are to pull back to the positions held before fighting broke out August 7 in South Ossetia.

Nogovitsyn said the Russian troops are pulling back to South Ossetia and a security zone defined by a 1999 agreement of the "joint control commission" that had been nominally in charge of South Ossetia since it split from Georgia in the early 1990s.

Georgian and Russian officials could not immediately clarify the dimensions of the security zone.

The United States has called for an emergency meeting of NATO on Tuesday to discuss the alliance''s worsening relationship with Russia, and U. S. Secretary of State Condoleezza Rice flew Monday to Europe to press the American viewpoint.

Rice said Russia can''t use "disproportionate force" against its neighbor and still be welcomed into the halls of international institutions.

For his part, President Medvedev said: "If someone thinks that our citizens can be killed, soldiers and officers who are peacekeepers killed with impunity, we will never permit that." (ANI)