Russian President Vladimir Putin was showing no signs of heeding Western calls to ease the standoff in Crimea, where pro-Kremlin forces stepped up their takeover of the Ukrainian region as it prepares for a separatist referendum.

Gunmen fired warning shots Saturday as international observers tried to enter Crimea for a third day and a Ukrainian border patrol plane came under fire that didn’t cause injuries.

The Associated Press reported that dozens of military trucks transporting heavily armed soldiers rumbled over Crimea’s rutted roads Saturday as Russia reinforced its armed presence on the disputed peninsula in the Black Sea.

Some of the army green vehicles had Russian licence plates and numbers indicating they were from the Moscow region. Some towed mobile kitchens and what appeared to be mobile medical equipment.

Vladislav Seleznyov, a Crimean-based spokesman for the Ukrainian armed forces, said, “Neither the equipment nor the paratroopers have insignia that identify them as Russian, but we have no doubt as to their allegiance.”

The Russians have denied their armed forces are active in Crimea, but an Associated Press reporter trailed one military convoy Saturday afternoon from 40 kilometres west of Feodosia to a military airfield at Gvardeiskoe north of Simferopol, over which a Russian flag flew.

In another development, an international military mission composed of officers from the U.S. and 28 other nations tried again Saturday to enter Crimea to monitor events for a third day, but it was turned back around the town of Armiansk by armed men.

An AP reporter travelling with the 54 observers from the Organization for Security and Co-operation in Europe said that after the group had stopped, the armed men fired bursts of automatic weapons fire to halt other unidentified vehicles. No injuries were reported.

In the week since Russia seized control of Crimea, Russian troops have been neutralizing and disarming Ukrainian military bases here. Some Ukrainian units, however, have refused to give up.

The new Crimean prime minister, Sergei Aksyonov, a little-known local businessman with a murky past and the nickname of “Goblin,” has said pro-Russian forces numbering more than 11,000 now control all access to the region and have blockaded all Ukrainian military bases that haven’t yet surrendered.

In Simferopol, Aksyonov attended a public ceremony for the swearing-in of the first unit in the pro-Russia “Military Forces of the Autonomous Republic of Crimea.” About 30 men armed with AK-47s, and another 20 or so unarmed, ranged in age from teens to a man who looked to be about 60, turned up at a park.

Aksyonov — who was greeted by them with shouts of “Commander!” — said their main role, at least until the referendum, would be to “keep the peace.”

U.S. Secretary of State John Kerry, meanwhile, warned his Russian counterpart Saturday that steps by the Kremlin to annex Crimea would “close any available space for diplomacy,” a U.S. State Department official said.

His warning came after leaders of Russia’s parliament said they would support a move by Crimea to break away from Ukraine and become part of the Russian Federation.

Washington has sought to establish a “contact group,” which would include Russia, Ukraine, Britain, France and the U.S., as a way to bring Moscow and Kyiv to the negotiating table, and Ukraine’s new foreign minister, Andrii Deshchytsia, said Saturday that a little progress had been made to form such a group.

Russian Foreign Minister Sergei Lavrov, however, ruled out any dialogue with Ukraine’s new authorities, whom he dismissed as the puppets of extremists.

“We are ready to continue a dialogue on the understanding that a dialogue should be honest and partner-like, without attempts to portray us as one of the parties in the conflict,” he said Saturday.

Ukraine is struggling to keep hold of Crimea, home to Russia’s Black Sea Fleet, after pro-Russian forces took control of it in the wake of Moscow-backed Viktor Yanukovych’s ouster as president.

Western officials say they’re concerned that the situation in the peninsula, where the U.S. estimates there now are 20,000 Russian troops confronting a smaller Ukrainian military force, threatens to explode at any moment.

“Russia and Ukraine, right now, are one nervous 20-year- old soldier’s mistake away from something very, very bad happening that could spin out of control,” said Steven Pifer, a former U.S. ambassador to Ukraine. There are about 12,000 Ukrainian troops in Crimea, he said.

The peninsula, where Russian speakers comprise a majority, will join Russia once parliament in Moscow passes the necessary legislation and there’s nothing the West can do to stop the process, according to Sergei Tsekov, the deputy speaker of Crimea’s parliament.

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“There’s no comeback, and the U.S. or Europe can’t impede us,” Tsekov said by phone from Moscow, where he met Russian officials to discuss the region’s future. “Crimea won’t be part of Ukraine any more. There are no more options.”

Seleznyov, the Ukrainian military spokesman, told AP that witnesses had reported seeing amphibious military ships unloading around 200 military vehicles in eastern Crimea on Friday night after apparently having crossed the Straits of Kerch, which separates Crimea from Russian territory.

The amphibious operation appeared to be one of the largest movements of Russian military forces since they appeared in Crimea a week ago.

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