MOSCOW, Russia, March 19, 2012  -- 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.

The Iman replaced another Russian ship "which had been sent to Syria for demonstrating (sic) the Russian presence in the turbulent region and possible evacuation of Russian citizens," the Black Sea Fleet told Interfax.

RIA Novosti, a news outlet with strong ties to the Kremlin, trumpeted the news in a banner headline that appeared only on its Arabic language website. The Russian embassy to the U.S. and to the U.N. had no comment, saying they have "no particular information on" the arrival of a Russian anti-terrorism squad to Syria.

Moscow has long enjoyed a cozy relationship with the Assad regime, to which it sells billions of dollars of weapons. In return Russia has maintained a Navy base at Tartus, which gives it access to the Mediterranean.

Last week Russia's Foreign Minister Sergei Lavrov said Russia had no plans to send troops to Syria.

"As for the question whether I consider it necessary to confront the United States in Syria and ensure our military presence there… in order to take part in military actions -- no. I believe this would be against Russia's national interests," Lavrov told lawmakers, according to RIA Novosti.

Russia's Defense Minister Anatoly Serdyukov denied reports that Russian special forces were operating inside Syria. He did say, however, that there are Russian military and technical advisors in the country.

U.S. State Department spokesperson Victoria Nuland said the U.S. government had not heard of the reports of Russian troops in Syria and declined to comment.

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