Syrian forces shelled the central city of Homs on Monday during a fierce offensive to root out rebels as the country's most important ally, Russia, appeared to show growing concern over President Bashar Assad's future.

Russia's Interfax news agency reported that two Russian navy ships are prepared to head to Syria to protect Russian citizens and a naval base there.

"We must protect our citizens," Maj.-Gen. Vladimir Gradusov was quoted as saying. "We won't abandon the Russians and will evacuate them from the conflict zone, if necessary."

Each ship is capable of carrying up to 300 marines and a dozen tanks, according to Russian media reports. That would make it the largest known Russian troop deployment to Syria, signaling that Moscow is becoming increasingly uneasy about Syria's slide toward civil war.

Russia has been Syria's strongest ally over the course of the uprising, which began in March 2011 with mostly peaceful protests against Assad's regime. A ferocious government crackdown led many to take up arms, and the conflict is now an armed insurgency.