An U.S.-Russian memorandum on mutual understanding in preventing incidents during operations in Syria does not 'constrain' Turkey's right to defend its airspace, press secretary of the U.S. Embassy in Russia William Stevens told Interfax.

"The U.S.-Russia MOU covers safety procedures between Coalition and Russian aircraft conducting counter-ISIL operations in Syrian airspace. It does not address or constrain Turkey's right to protect its sovereign airspace in accordance with international law. We support Turkey's right to defend its territory and airspace," Stevens said.

Turkey informed the United States and NATO on November 24 that its military airplane shot down a Russian Sukhoi Su-24 fighter "that had violated Turkish airspace."

"As [U.S.] President [Barack] Obama has said, it is very important that the Russians and the Turks talk to each other and take measures to discourage any kind of escalation. President Obama spoke to [Turkish] President [Recep Tayyip] Erdogan Monday to personally convey this message, and [U.S.] Secretary [of State John] Kerry has done the same yesterday with [Russian Foreign] Minister [Sergei] Lavrov. We were encouraged that the Turkish and Russian foreign ministers have subsequently spoken," the U.S. diplomat said.

It was reported earlier that Russian Foreign Minister Sergei Lavrov wondered if Turkey asked the U.S. for permission to use an F-16 jet fighter to attack a Russian Sukhoi Su-24 bomber.

"Some coalition members, including those that have allocated their warplanes for delivery of airstrikes in Iraqi and Syrian territory, have shared it with us confidentially that, as the planes they have on their inventory are U.S.-made, the Americans demanded that, before delivering airstrikes, these coalition members ask the U.S. for consent for such an action," Lavrov said at a press conference in Moscow on Wednesday.

He pointed out that the Russian plane was shot down by a U.S.-made F-16 jet fighter.

"I wonder whether this demand by the Americans applies to Turkey. And if it does, I wonder whether Turkey had asked permission from the U.S. to scramble its planes and shoot down a plane over Syrian territory, even if [it was] an unidentified one," Lavrov said.