A Russian military unit has arrived in Syria, according to Russian news reports, a development that a United Nations Security Council source told ABC News was "a bomb" certain to have serious repercussions.

Russia, one of President Bashar al-Assad's strongest allies despite international condemnation of the government's violent crackdown on the country's uprising, has repeatedly blocked the United Nations Security Council's attempts to halt the violence, accusing the U.S. and its allies of trying to start another war.

Now the Russian Black Sea fleet's Iman tanker has arrived in the Syrian port of Tartus on the Mediterranean Sea with an anti-terror squad from the Russian Marines aboard according to the Interfax news agency. The Assad government has insisted it is fighting a terrorist insurgency. The Russian news reports did not elaborate on the Russian troops' mission in Syria or if they are expected to leave the port.

The presence of Russian troops in Syria could be a "pretty obvious" show of support to the regime, according to Russian security expert Mark Galeotti.

"No one thinks of the Russians as anything but Assad's last friends," said Galeotti, professor of global affairs at New York University.

More here: http://abcnews.go.com/Blotter/russian-anti-terror-troops-arrive-syr...