The U.S.-made long-range missile systems sent to Ukraine were secretly modified to prevent strikes against Russia, a report said Monday.

The decision on the High Mobility Artillery Rocket Systems (HIMARS) was taken to "reduce the risk of wider war with Moscow," the Wall Street Journal reported, citing anonymous U.S. officials.

Washington has sent 20 HIMARS to Kyiv since June, it said. The systems were widely credited for helping Ukraine make unexpected gains in fighting off Russian forces.

The Pentagon has yet to make any statement regarding the reports.

The U.S. in late November announced an additional $400 million in military assistance to Ukraine to help the country defend itself against Russia.

The new package includes additional arms, munitions, and air defense equipment, including additional HIMARS, according to the US Defense Department.

The U.S. has so far committed more than $19 billion in security assistance to Ukraine since the beginning of the war on Feb. 24.

"Ukrainian President Volodymyr Zelenskyy formally promised only to use HIMARS for defensive purposes and to avoid firing into Russian territory; this took place before the U.S. agreed to provide the systems in order to avoid escalating the conflict," Defense News reported this July.