DONETSK, Ukraine -- Defiant pro-Russian militants in eastern Ukraine pushed the country to the brink of war or dissolution Monday, expanding their hold, while the acting president failed to make headway in trying to end the crisis.
After an ultimatum to the militants was ignored, acting President Oleksandr Turchynov first vowed to rout them by force, then held out the offer of a referendum to decide Ukraine's fate, then proposed a peacekeeping intervention by the United Nations.
Nothing Mr. Turchynov said moved the pro-Russian forces, who seized another police station in another small town, Horlivka.
In a nation of 44 million, it became clear that a few hundred men, operating on the eastern fringes of the country with guns and unmarked uniforms, have brought Ukraine to a deeply dangerous juncture.
The mood was tense in Donetsk, an industrial city of nearly 1 million, where many residents were staying inside after dark. Pro-Russian activists took over the regional administrative offices last week, and bands of masked men, including several carrying steel pipes, were patrolling the barricaded entrances to the monolithic structure in the center of town.
Mr. Turchynov and other Ukrainian officials are sure that Russia is guiding the militants as they have steadily taken over one government building after another. They have vocal support on that score from Washington and London. Russia adamantly denies it, and Russian Foreign Minister Sergei Lavrov said Monday that it is the West's responsibility to rein in the Kiev government, so that there are no violent attacks on the militants.
The crisis, which began to the south, in Crimea, is now focused on militants who say they represent the "People's Republic of Donetsk." It has brought relations between Russia and the West to their lowest point at least since the Soviet invasion of Afghanistan in 1979.
"There can't really be any real doubt that this is something that has been planned and brought about by Russia," said British Foreign Secretary William Hague, in Luxembourg to meet with European counterparts.
In Moscow, a spokesman for Vladimir Putin said the Russian president has been watching the crisis with "great concern" and had received "many appeals, addressed personally to Putin, asking to help in this or that way and asking to intervene in this or that way."
Pentagon officials on Monday protested what they described as a provocative flyover by a Russian attack aircraft that flew at close range for 90 minutes over a U.S. Navy ship that had been sent into the Black Sea.
The anonymous appeal for help has been a favorite tactic of Russian interventionists for the better part of a century. It was rolled out before the invasion of Hungary in 1956, and of Czechoslovakia in 1968 -- two operations in which one of Mr. Putin's heroes, Yuri Andropov, a KGB official who later became Soviet leader, played a key role. It was also a feature of Russia's involvement in Crimea in late February and March, before that region's annexation by Moscow.
In eastern Ukraine, Ukrainian news agencies reported Monday evening that foes of the separatists had set up checkpoints on highways leading from the Donetsk region to the Kharkiv region, and -- with help from traffic police -- were inspecting cars to prevent separatists from traveling to Kharkiv.
But as the evening wore on, there was still no sign of Mr. Turchynov's promised attack on separatist positions by forces loyal to Kiev. Mr. Turchynov and other officials had said that if no resolution was reached by 8 a.m. Monday, an "anti-terrorist" operation would begin.
In the heart of Donetsk, a group of pro-Russians occupying the city's Lenin Square said they were convinced that the Kiev government would not carry out its threat to deploy the Ukrainian army in the region. Unlike some activists, they said they did not want the Russian military to roll into the region. They were holding out for a referendum on the region's future.
"We are not afraid," said retired coal miner Oles Kulik, occupying a tent in the square where banners declaring the People's Republic were plastered on lampposts. "Kiev needs to hear our voice and understand that we are now something different from them."
Elsewhere in the region, the several hundred pro-Russian militants have established themselves in towns on strategic crossroads, but wouldn't be able to withstand an armored assault -- unless such an attack draws assistance from Russian military forces right across the border, which is exactly what Kiev and its friends abroad fear.
This being Ukraine, though, some people -- including Oleksandr Yaroshenko, a veteran political strategist in Donetsk -- worry that Mr. Turchynov and his ally, former Prime Minister Yulia Tymoshenko, are gaming the crisis for personal benefit and may not intend to resist Russia to the fullest.
One factor at play is the upcoming presidential election, scheduled for May 25 but looking increasingly unlikely. On Monday, Mr. Turchynov unexpectedly held out the offer of a referendum to determine the amount of power-sharing between Kiev and the regions, to be held the same day. The separatists have been saying a referendum is their principal goal, but many were quick to dismiss Mr. Turchynov's proposal as insincere -- as was the Russian foreign ministry.
Mr. Turchynov also spoke with United Nations Secretary-General Ban Ki-moon and later suggested that a U.N. peacekeeping force could enter eastern Ukraine. But the almost-certain opposition of Moscow, standing up for the men with guns, makes that problematic.
U.S. Secretary of State John Kerry and European leaders are promising more sanctions, beyond those imposed on Russia over its Crimea action.