BEIRUT  - Russia sent warships to the Mediterranean to prepare a potential evacuation of its citizens from Syria, a Russian news agency said on Tuesday, a sign President Bashar al-Assad’s key ally is worried about rebel advances that now threaten even the capital.Moscow acted a day after insurgents waging a 21-month-old uprising obtained a possible springboard for a thrust into Damascus by seizing the Yarmouk Palestinian camp just 2 miles from the heart of the city, activists said.The anti-Assad opposition has posted significant military and diplomatic gains in recent weeks, capturing a series of army installations across Syria and securing formal recognition from Western and Arab states for its new coalition.Assad’s pivotal allies have largely stood behind him. But Russia, his main arms supplier, appeared to waver this week with contradictory statements repeating opposition to Assad stepping down and airing concerns about a possible rebel victory. Russia’s Interfax news agency quoted unnamed naval sources on Tuesday as saying that two assault ships, a tanker and an escort vessel had left a Baltic port for the Mediterranean Sea, where Russia has a port in Syria’s coastal city of Tartus.“They are heading to the Syrian coast to assist in a possible evacuation of Russian citizens ... Preparations for the deployment were carried out in a hurry and were heavily classified,” the Russian agency quoted the source as saying.It was not possible to independently verify the report, which came a day after Russia confirmed that two citizens working in Syria were kidnapped along with an Italian citizen.