U.S. officials have uncovered an effort by Iran to help Syria mask its oil exports and evade an American and European embargo, in a potent new sign of Tehran's campaign to bolster Syrian President Bashar al Assad as his regime cracks down on public opposition.

American officials investigating the Iranian operation said it was designed to quietly ship Syrian crude oil to Iran, where it can be sold on the international market, with revenue going back to Damascus. Transit records document one such shipment -- involving more than 100,000 tons of crude -- which took place last month.

"The oil shipment to Iran was designed to evade the sanctions that have been imposed on Syria," said a senior U.S. Treasury Department official familiar with the case.

In response, the treasury department has begun targeting the insurance and registration of international tankers shipping Syrian oil overseas. Many are insured in the U.S.

Concerns aren't limited to Iran. Washington and its allies are also intensifying the scrutiny of maritime and air traffic moving into Syria from Russia, as Moscow has publicly committed to continue arming Assad's security forces.

This month, Cyprus intercepted a St. Petersburg-based ship, the Chariot, that was moving four containers of munitions bound for the Syrian port of Tartus, according to Cypriot officials. Cyprus eventually released the ship after assurances from its Russian owners that it wouldn't complete the delivery, according to Cypriot officials.

But Moscow this week confirmed the arms shipment was made. The ship's owner, the Russian freight company Balchart, declined to comment.

A spokesman for the Iranian embassy at the UN said there are no international sanctions on Syria that Tehran needs to respect. He also said that outside powers, not Iran, have been feeding the conflict inside Syria by shipping in arms. "Syria is an independent country, and Iran respects its sovereignty," said the spokesman. "Iran believes that Syrians have the right to self-determination free from any foreign intervention."

Click here to read more on this story from The Wall Street Journal.