In a letter to the White House on Tuesday, four Republican members of Congress encouraged U.S. President Joseph Biden to send cluster munitions to Ukraine, claiming that the government is afraid that doing so will be viewed by Moscow as an escalation.

According to a report from Reuters earlier this month, Ukraine is searching for the MK-20, an air-delivered cluster bomb, to release its individual explosives from drones and 155 mm artillery cluster shells. Congressmen had been asked by Kiev to put pressure on the White House to permit the shipment of the weaponry.

The letter criticized Biden for “reluctance to provide Ukraine the right type and amount of long-range fires and maneuver capability to create” a breakthrough against Russian forces.

The letter was signed by Jim Risch, the top Republican on the Senate Foreign Relations Committee, Roger Wicker, the top Republican on the Senate Armed Services Committee, Mike McCaul, the chairman of the House Foreign Affairs Committee, and Mike Rogers the chairman of the House Armed Services Committee.

It urged Biden to send the Dual-Purpose Conventional Improved Munitions (DPICM) found in several types of U.S. munitions, including 155 millimeter artillery, GMLRS and ATACMS.

It is by no means certain the Biden administration would sign off on a transfer.

Cluster munitions, banned by more than 120 countries, normally release large numbers of smaller bomblets that can kill indiscriminately over a wide area, threatening civilians.