Russia has promised to retaliate if the United States imposes new sanctions for Moscow’s brutal bombing campaign in Syria, especially against the besieged town of Aleppo.

In fact, as the UK Independent observes, Moscow threatened “asymmetric” retribution, which sounds ominous.

The threats came from Deputy Foreign Minister Sergei Ryabkov, who also complained to the Russian Federation Council’s international affairs committee about how “the sanctions policy of the U.S. towards Russia has continued for a long time.”

“We have used this period to conduct certain research and prepare a series of measures that will be applied in an asymmetric way should the sanctions be toughened further,” said Ryabkov.

Russia’s last response to U.S. sanctions was to suspend an agreement on the disposal of plutonium which dated back to the year 2000. Ryabkov said that disposal program will not be reactivated unless the United States “ends its sanctions and decreases its military presence in Europe.”

For the moment, the possibility of fresh American sanctions remains hypothetical, as Secretary of State John Kerry is pursuing diplomatic channels with his Russian opposite number, Sergei Lavrov, to secure a longer humanitarian bombing pause.

Bloomberg News notes one of the big problems with U.S. sanctions threats is that Washington is “stuck with nobody left to sanction in Russia.”

The article quotes Michael Kofman of the Wilson Center’s Kennan Institute suggesting the Kremlin’s janitor might be the only Russian who is not already under sanctions. There are even sanctions in place against a Russian motorcycle gang because they allegedly helped Russia against Ukraine.

The most realistic option for further punishment would appear to be sanctions against individual Russian military officers and weapons suppliers involved in the Syrian campaign, although Bloomberg notes blocking such persons and entities from visiting the Unites States or dealing with U.S. banks is not that much of a hardship. Also, the time necessary to process and impose new sanctions might be too long to help the residents of Aleppo in any meaningful way.

The existing sanctions regime arguably has damaged the Russian economy, although the pain might have been harder to feel over the agony inflicted by cratering oil prices. There is little sign of the Kremlin changing its behavior in any theater as a result of sanctions.

More dramatic options for further sanctions seem to be off the table because they would clobber Russia’s European customers for energy harder than they would hit Moscow. The European Union recently passed on an opportunity to sanction Russia over Aleppo, but held out the possibility they would consider such measures if the carnage continues.

NATO officials believe Russia is planning to escalate bombing in Aleppo even further, undeterred by reports that the latest airstrike in Idlib province killed more than a dozen schoolchildren.

In recent action at the United Nations, Under-Secretary General for Humanitarian Affairs Stephen O’Brien declared himself “incandescent with rage” for the Security Council’s failure to halt the killing in Syria, saying it would become “this council’s legacy, our generation’s shame.” He called Aleppo a “kill zone,” citing the number of children who have been killed.

Russia’s U.N. ambassador, Vitaly Churkin, responded by saying O’Brien’s description of the humanitarian disaster in Aleppo was “unfair and dishonest,” and suggested he hold on to the heated prose for “the novel [he is] going to write someday.”