DONETSK, Ukraine — Defiant pro-Russia militants in eastern Ukraine pushed this 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, the 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 Turchynov said moved the pro-Russia forces, who seized another police station in another small town, Horlivka. Over the past 10 days, they have taken over more than a dozen government offices in eastern Ukraine.
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 this industrial city of nearly 1 million, where many residents were staying inside after dark. Pro-Russia 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.
Turchynov and other Ukrainian officials said they are convinced 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 the Russian foreign minister, Sergey Lavrov, said Monday it is the West’s responsibility to rein in the government in Kiev so that there are no violent attacks on the militants. Russia has tens of thousands of troops massed on the Ukrainian border.
In Washington, a senior administration official said President Obama told President Vladimir Putin of Russia in a phone call Monday that while a diplomatic solution to the crisis remains open, Russia’s actions have not been conducive to that approach, the Associated Press reported.
The official said Obama told Putin that Ukraine’s central government has made real offers to address concerns about giving local governments more power. But the official said Obama reiterated his belief that Ukrainians must decide those matters.
Secretary of State John Kerry and European leaders are promising more sanctions, on top of those imposed on Russia over its role in Crimea. The State Department circulated a document assailing what it called ‘‘Russian Fiction: The Sequel. Ten More False Claims about Ukraine.’’
In seeking to contradict assertions from Moscow, the American document says, among other things, that Russian agents are active in eastern Ukraine; that separatists there do not enjoy broad popular support; that Russian speakers are not under threat; and that the new government in Kiev is not led by right-wing fascists.
The crisis, which was exacerbated in 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,’’ the British foreign secretary, William Hague, said as he arrived in Luxembourg to meet with his European counterparts.
Turchynov talked with UN General Secretary Ban Ki-moon on Monday and later suggested that a UN peacekeeping force could enter eastern Ukraine. But that would almost certainly draw a veto by Russia.
In Moscow, a Putin spokesman said the president has been watching the crisis with ‘‘great concern’’ and had received many appeals asking that he intervene.
The anonymous appeal for help has long been a favorite tactic of Soviet and Russian interventionists. It was rolled out before the invasion of Hungary in 1956 and of Czechoslovakia in 1968.
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 that opponents of the separatists had set up checkpoints on highways leading from the Donetsk region to the Kharkiv region, and — with the help of traffic police — were inspecting cars with the aim of preventing separatists from traveling to Kharkiv.
But as the evening wore on there was still no sign of Turchynov’s promised attack on separatist positions by forces loyal to Kiev. Turchynov and other officials had said that if no resolution was reached by 8 a.m. Monday, an ‘‘antiterrorist’’ operation would begin.