Senior Republicans on the House Foreign Affairs Committee wrote in a letter to Secretary of State Tony Blinken on Wednesday that the administration is obligated, under pre-existing U.S. law, to levy additional sanctions against Iran in response to its support for Russia’s invasion of Ukraine and the deepening relationship between the two U.S. foes.

The letter, from HFAC Chair Mike McCaul (R-TX), Middle East, North Africa and Central Asia Subcommittee Chair Joe Wilson (R-SC) and Europe Subcommittee Chair Tom Kean (R-NJ) represents the latest effort by congressional Republicans to push the administration to tighten sanctions on Iran.

The letter says that Iran’s sale of weapons to Russia should trigger additional mandatory sanctions on Iran under the Countering America’s Adversaries through Sanctions Act (CAATSA), passed by Congress in 2017 to crack down on Russia, Iran and North Korea. The sanctions bill passed both chambers of Congress by nearly unanimous votes. Congressional Democrats later accused the Trump administration of failing to fully implement anti-Russia sanctions.

“Building upon years of coordination in Syria, we are now witnessing the disturbing emergence of ‘a full-fledged defense partnership’ between Russia and Iran, resulting in concerning advances in Iran’s military capabilities,” Wednesday’s letter reads. “Faced with this emerging threat, our partners and allies — especially those that perceive Iran as a threat to their national security — must help ensure Ukraine receives the critical weapons systems it needs to secure a decisive victory.”

The letter highlights Iran’s provision of drones, as well as bullets, rockets and mortar shells, to Russia for its war and concerns that Iran may further escalate its assistance to include ballistic missiles. It also notes that Iran has claimed it is set to receive advanced Russian Su-35 jets by the end of the year.

“Russia advancing the Su-35 transfer now alarmingly suggests Russia has made a strategic decision that Iran will be its uncontested military-technical partner of choice in the region,” the lawmakers wrote.

CAATSA requires the president to impose sanctions on entities that knowingly engage in a “significant transaction” with Russia’s defense or intelligence sectors. Such sanctions have previously been triggered against Turkey for its purchase of Russian S-400 air defense systems.

“The United States must leverage all relevant policy tools, including export controls and sanctions authorities, to discourage and hamper the destabilizing relationship between these two dangerous malign actors,” the letter reads. “The growing threat posed by Russia and Iran’s partnership requires an urgent response. In addition to sanctions and export controls, we urge you to engage in robust diplomacy with allies and partners to ensure they understand the significance and the threat of this unholy alliance.”