Russia has apparently denied Iran’s request to acquire the S-400 missile system amid the Iranian threat, reported Bloomberg on Thursday, citing two people familiar with the situation, “including a senior Russian official.”

Iranian Foreign Minister Javad Zarif was in Moscow earlier this month.

Thursday’s development comes as tension between Iran and the United States, and its Gulf partners, has increased in the past month.

The United States enacted new sanctions and deployed two warships with fighter jets, in addition to a Patriot missile battery, to the Gulf in response to Pentagon reports that Iran’s Islamic Revolutionary Guard Corps was planning an attack on U.S. forces or interests in the region

However, the Kremlin has denied Russian President Vladimir Putin’s response, according to Anna Borshchevskaya of the Washington Institute for Near East Policy.

“Kremlin propaganda outlet Sputnik has claimed that Moscow and Tehran haven’t had the discussion about the S-400 to begin with,” she told JNS. “I’ve maintained over the years that Russia and Iran are not going to split anytime soon, and I still maintain that. They have their differences, it’s a complicated relationship, but their common larger goals of anti-Americanism continue to hold them together. This developing S-400 story is important to watch to be sure, but I don’t think it’s a signal of any real split between Russia and Iran.”