WASHINGTON (Reuters) - The United States is working with Egypt to deter it from going ahead with a deal to buy Russian warplanes, threatening the Cairo government with sanctions if it does, a senior State Department official said on Thursday.

Washington was still working with Egypt to address its defense needs, the official, speaking on the condition of anonymity, told a briefing. “But we’ve also been transparent with them in that if they are to acquire a significant Russian platform ... that puts them at risk towards sanctions.”

The United States has over the years provided billions of dollars in economic and military aid to Egypt, a long-time ally whose military operates the F-16 fighter jet.

Egypt earlier this year signed a $2 billion agreement with Russia to buy more than 20 Su-35 fighter jets. The United States could impose sanctions under the Countering America’s Adversaries Through Sanctions Act (CAATSA), which targets purchases of military equipment from Russia.

“They know this and we’re working through it with them,” the U.S. official said. “This is something we’ve not completely reconciled yet but they’re acutely aware of what they’re putting at risk.”

The United States says using the Su-35 and other Russian weapons systems could pose a threat to a country’s ability to operate jointly with the militaries of the United States and other NATO countries.

Washington has also made similar threats to Turkey, which has already purchased Russian defense missile systems but has not yet made them operational.