A bipartisan group of senators urged President Joe Biden to strengthen oil sanctions on Iran, with a particular eye toward targeting Chinese purchases of Iranian oil, which have been a lifeline for the regime in Tehran.

“In the wake of the October 7 terror attacks and subsequent attacks by Iran-backed proxies on U.S. forces in the Middle East, we urge you to work harder to stop Iran’s funneling of lucrative oil exports to finance terror,” the lawmakers wrote in a letter to the president. “These exports provide a crucial lifeline to sustain and expand Iran’s sponsorship of terrorist groups whose objective is to harm U.S. interests in the Middle East and to destroy our ally, Israel.”

The lawmakers specifically called on the administration to impose additional sanctions on ships transporting Iranian petroleum, as well as the ports and refineries that process this oil. The majority of Iran’s oil exports are purchased by China.

The letter references two bills — the SHIP Act and the End IT Act — which both seek to impose additional sanctions on China for its purchases of Iranian oil, calling on the administration to “use all available resources to proactively implement these sanctions while Congress considers enacting legislation.”

The letter was signed by 18 lawmakers, five of whom are Democrats.

Sens. Marco Rubio (R-FL) and Maggie Hassan (D-NH) led the letter, joined by Sens. Jim Risch (R-ID), Jacky Rosen (D-NV), Mike Crapo (R-ID), Bob Casey (D-PA), Bill Hagerty (R-TN), John Fetterman (D-PA), Shelley Moore Capito (R-WV), Catherine Cortez Masto (D-NV), Lisa Murkowski (R-AK), Marsha Blackburn (R-TN), Rick Scott (R-FL), Katie Britt (R-AL), Deb Fischer (R-NE), Ted Budd (R-NC), John Cornyn (R-TX) and Pete Ricketts (R-NE).

A bipartisan group of 62 House members sent a similar letter to the administration earlier this week.