All this leaves Ukraine largely on its own to fight off a well-coordinated campaign of attacks on government buildings that it denounces as Russian aggression but that it has so far been unable to prove incontrovertibly involves Russian soldiers, agents or money. Having already lost Crimea, Mr. Turchynov, the acting president, vowed on Sunday that “we will not let Russia repeat the Crimean scenario in the eastern region of Ukraine.”

But if Russia is indeed now seeking to repeat its Crimea seizure, something that Moscow has repeatedly denied, Ukraine’s defenses are fragile.

So, too, is the government amid rising public anger at its inability to keep the country together. After another day of bleak news from the east, protesters set tires on fire late Monday outside the Parliament in Kiev. They demanded that the acting interior minister, Arsen Avakov, resign and that Mr. Turchynov explain why things have gone so wrong.

The SBU, under new leadership since Ukraine’s February revolution, has repeatedly boasted of catching people suspected of being Russian operatives in the east, but it has not yet made public any solid evidence to support Kiev’s assertions that the mayhem in Donetsk and neighboring provinces has been orchestrated and financed by Moscow. This has left the Russian foreign minister, Sergey V. Lavrov, free to taunt Kiev and the West for making accusations they have not yet substantiated. It has also compromised Ukraine’s efforts to compete, at least in eastern Ukraine and in Russia itself, with Moscow’s own narrative of Western meddling.

Alfa, by contrast, does appear to be more or less functioning, but its officers, bitter at being challenged over their role under Mr. Yanukovych, do not understand when “you tell them one day that they are murderers and then send them the next day to free a building” in Donetsk, said Serhiy Skorokhvatov, a former officer in the force and president of its veterans association.

Like other police and special forces units, Alfa took part in what Mr. Yanukovych called an “antiterrorist operation” and worked to crush the pro-European protests that brought the current government to office. Oleh Prysizhniy, who headed Alfa until Mr. Yanukovych fled, is under investigation by both Parliament and the prosecutor general for the unit’s possible role in killing protesters.