The U.S. is sending Ukraine some Soviet-made air defense equipment that Washington took charge of decades ago through a secret program, The Wall Street Journal reported on Monday.

The systems, to include the SA-8 short-range surface-to-air missile system, were obtained by the U.S. for the purposes of examining Russian military technology and helping train American troops, U.S. officials told the outlet.

The weapons are useful to Ukrainian forces, as their military already knows how to use Soviet systems.

Both the National Security Council and Pentagon declined to comment on what specific weapons the U.S. has sent to Ukraine to help the country beat back a violent Russian invasion that began Feb. 24.

“Operational security matters to the Ukrainians, right now,” press secretary John Kirby told reporters Monday.

“They’re fighting for their country, and the Pentagon is not going to be detailing publicly the tools with  which they are doing that,” he added.

The Biden administration has approved more than $1 billion in military aid to Ukraine in the past month, including an $800 million package announced last week.

But the U.S. government has been hesitant to detail exactly what is being sent in so as not to tip off or draw the ire of Moscow. The Kremlin has publicly stated that any Western country that provides certain weapons to Ukraine, including aircraft and missile defense systems, could be seen as entering the fight.

The U.S. has a small number of Soviet missile defense systems it acquired in the past 30 years as part of a secret, $100 million project that first gained notice in 1994, a former official involved in the mission told the Journal.

Among the weapons the U.S. received — some of which have been kept at Redstone Arsenal, Ala. — is the SA-8, which can be easily moved with ground forces and provide cover from aircraft and helicopters.