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, 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-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 this industrial city of nearly one 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.

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 the Russian foreign minister, Sergei 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.

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," the British foreign secretary, William Hague, said as he arrived in Luxembourg to meet with his 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."

Officials at the Pentagon 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 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 its annexation by Moscow.

In eastern Ukraine, Ukrainian news agencies reported Monday evening 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.