The United States and its allies are concerned that they could be too late to detect a small Russian nuclear strike on Ukraine, Politico reported Tuesday, citing five unnamed current and former U.S. officials.

Russia has 23 dual-use weapon systems that can deliver both conventional and low-yield nuclear warheads, one official was cited as saying. Many of them have been used with conventional munitions in Ukraine, according to Politico.

“Almost every single weapon the Russians have is nuclear-capable,” a former senior National Security Council official who still advises U.S. Strategic Command said.

They listed artillery systems, air defense systems, torpedoes and cruise missiles as the weaponry that could be equipped with nuclear warheads.

Unlike strategic arms such as intercontinental ballistic missiles, conventional weapons capable of carrying tactical nuclear warheads do not give off apparent signals when they are put on alert.

“For those smaller nuclear weapons, we’re probably not going to know,” a U.S. government official with access to intelligence on Russia’s nuclear forces and strategy was quoted as saying by Politico.

Concerns of a possible Russian tactical nuclear attack have been high in the week since President Vladimir Putin issued a thinly veiled threat to do so while announcing a call-up of reservists for battle in Ukraine. 

Putin also said he would approve the annexation of four Russian-occupied Ukrainian regions following referendums.

Analysts say Russia’s formal takeover of Ukrainian lands means Putin could authorize a limited nuclear strike in the event that advancing Ukrainian forces moved to retake those territories. Kyiv has said it would press on with its counteroffensive despite the annexation referendums, which it dismisses as a sham.

Putin’s ally and former President Dmitry Medvedev went a step further Tuesday, speculating that Russia could be “forced to use the most fearsome weapon” against Ukraine and voicing confidence that NATO would not directly interfere in the conflict.

Russia has an estimated 1,900 tactical, or non-strategic, nuclear warheads that one U.S. official interviewed by Politico referred to as “micro-nukes.”

“That’s everything from cruise missiles to nuclear torpedoes to gravity bombs to intermediate-range ballistic missiles,” the official said.

According to people interviewed by Politico, the U.S. might never know when conventional munitions are swapped for atomic counterparts unless Putin would want the world to know.

To plug the intelligence gap, Politico reported that U.S. and its allies have tasked air, space and cyber intelligence assets and deployed commercial satellites to analyze Russian units that could get nuclear orders.

U.S. Air Force surveillance planes have also been spotted circling Kaliningrad, Russia’s western exclave sandwiched between EU members Poland and Lithuania.

But former Pentagon official Franklin Miller said he believes Russia would “signal” to the U.S. its preparedness for a nuclear strike.

“They’d give us a hint that they’re moving munitions from central storage sites to firing unit. And then give us more time to think about it and worry,” Miller said.