The president has repeatedly said he intends to stay out of the Balkans war, but he revised that stance yesterday by adding an important new proviso: Russia will not get embroiled - "unless pushed".
He said on television: "I told Nato, the Americans, the Germans, don't push us towards military action. Otherwise there will be a European war, for sure, and possibly world war, which must not be permitted."
In separate comments, he warned that if Nato went through with what he considers its aim "to seize Yugoslavia and make it their protectorate", there could be a stronger response from Moscow.
He did not specify what, but went on: "We can by no means give Yugoslavia away." He said a ground war would mean "big losses", as the Serbs are prepared to fight to "the last man".
The president's outburst amounts to a strategic decision to step up the barrage of angry words flying westwards over the Kremlin's battlements since the conflict began.
So far Mr Yeltsin and his premier, Yevgeny Primakov, have sought to tread a precarious line in this conflict. They have made clear that they intend to refrain from sending weapons to Belgrade or joining the war militarily, knowing this would vastly complicate relations with the West.
But they have also maintained a noisy flow of complaints and diplomatic moves to appease a genuinely outraged public and - crucially - a dangerously exasperated and anti-western Russian military.
But they are under intense domestic pressure to do more. Almost every major political entity outside the enfeebled liberal democrats has called for Russia to arm the Serbs.
The now-emboldened Duma, parliament's Communist-dominated lower house, voted overwhelmingly to send weapons and military advisers. Mr Yeltsin is threatened with impeachment proceedings, although these face many hurdles. An investigation is underway into corruption within the Kremlin. And his restless, broken-down army is deeply humiliated by Nato's strike on old Slavic friends. He needs to get this right.
So far, Moscow has restricted its protests to considered manouevres. It has severed ties with Nato, dispatched a surveillance warship to the Adriatic, and sent 80 lorry-loads of humanitarian aid to Belgrade (only days after accepting EU and US food aid to Russia).
At the same time, Russia has tried to position itself as a mediator, knowing that whoever leads the world out of this deepening crisis will emerge with a hero's garlands.
Not unreasonably, Moscow's hopes are pinned on suspicions that Nato has been drawn into an unwinnable conflict.
Although it remains likely that the Russian government will continue to resist being drawn militarily into Yugoslavia's deadly vortex, Mr Yeltsin clearly believes his task is getting daily more difficult.
Yesterday he embraced suggestions that Yugoslavia might join the Belarus- Russia union - another symbolic gesture because the alliance between Moscow and Minsk has been singularly fruitless. Even in these dark hours, Russia will not want to be seen to be locked in a three-way embrace with Belarus's dictatorial Alexander Lukashenko and Mr Milosevic.
Nato and the Kremlin refused to comment on the most thespian Russian gesture of the day, the reported re- targeting of Moscow's nuclear missiles at Nato countries taking part in the bombing.
These originated from Gennady Seleznyov, the Duma's chairman, who said he and the president discussed the issue yesterday. Initially, claims shot around the world saying the warheads had already been redirected at Nato countries.
Though this was officially denied, during the diplomatic panic the British Embassy in Moscow was issued urgent instructions from London to check the information. Robin Cook, the Foreign Secretary, said nothing that Nato "is doing in Yugoslavia or Kosovo ... poses the remotest threat to Russia".
There could be "no justification" for Russia "increasing its military posture".
Last night, these assurances turned out to be premature.
But the fact that Mr Cook talks separately about Yugoslavia and Kosovo will do nothing to soothe Mr Yeltsin's ire. Or his nerves.