A bipartisan group of 36 U.S. Senators is pressing the European Union to immediately close “a significant loophole in U.S.-E.U. sanctions policy” as P5+1 leaders meet with Iranian officials in Kazakhstan.

In a letter to E.U. Council President Herman Van Rompuy, the Senator’s emphasize the need to restrict Iran’s access to the “Target2” system which it reportedly uses to convert its foreign-held euros into local currencies.

Barring direct or indirect access to the Target2 service by accounts held by the Iranian government or its affiliates would prevent Tehran from skirting the impact of sanctions put in place due to the regime’s intransigence regarding its nuclear program.

As stated in the letter, the U.S. and E.U. “need to maintain and increase pressure if diplomacy has a hope of succeeding” in dissuading Iran from advancing its nuclear capabilities. The letter was spearheaded by Senators Mark Kirk (R-IL) and Jeanne Shaheen (D-NH).