The United States has received warning of a Russian deal which would see the sale of ground-to-air missile systems to Syria, according to reports from The New York Times and The Wall Street Journal.
American officials told The Times that the US is worried about a Russian plan to sells S-300 missile batteries to bolster Syrian air defenses.
The system is regarded as highly effective and would limit the ability of the United States and other nations to operate over Syrian airspace or impose a no-fly zone.
The Times attributed the information to one senior US official and another American official, both of whom divulged the news anonymously.
The Journal said US officials had received information from Israel about the possible sale, "but wouldn't comment on whether they believed such a transfer was near."
"Russian officials didn't immediately return requests to comment," wrote The Journal. US officials said Syrian President Bashar al-Assad's government has been seeking to purchase S-300 missile batteries since the administration of former President George W. Bush.
The sale of such air-defense systems would make intervention in the Syrian civil war much more complicated.
Meanwhile, US Secretary of State John Kerry said Thursday that any political solution in Syria will have to include Assad stepping down.
Kerry was meeting with his Jordanian counterpart, Nasser Judeh, in Rome. He announced that an additional $100 million in humanitarian aid will be dedicated to Syrian refugees, especially the 525,000 currently estimated to be in Jordan.
Kerry was in Moscow earlier this week, and said while meeting with Russian Foreign Minister Sergey Lavrov that they would "seek to convene an international conference" on the Syrian conflict as soon as possible.