Kiev (AFP) – US Vice President Joe Biden on Tuesday warned Russia of isolation if it continues to try to “pull Ukraine apart” and pledged Washington’s strong support for Kiev’s leaders, as a Cold War-style confrontation over the former Soviet republic ratcheted up.

“We call on Russia to stop supporting men hiding behind masks in unmarked uniforms, sowing unrest in eastern Ukraine,” Biden said as he was wrapping up a two-day visit to Kiev.

“We have been clear that more provocative behavior by Russia will lead to more costs and to greater isolation,” said the vice president, who flew back to Washington hours later.

Biden’s warning came as signs on the ground made it clear that diplomacy was failing to calm the crisis — opening the door for more threatened U.S. sanctions on Moscow.

Overnight Monday, pro-Kremlin rebels in Ukraine’s east claimed control of the police station in the town of Kramatorsk, where they already occupied the town hall.

The Organization for Security and Cooperation in Europe, which has more than 100 monitors in Ukraine, said the station’s police chief was abducted by the militants and called for his release.

In a statement, the OSCE chief monitor in the country, Ertugrul Apakan, condemned such “provocative actions,” saying they “can only worsen the existing tensions and contribute to further violence.”

Ukraine’s acting president, Oleksandr Turchynov, said in his own statement that the new seizure “puts a cross through all the agreements reached in Geneva.”

He referred to an accord signed last Thursday by Ukraine, Russia, the United States and Europe designed to de-escalate the volatile situation and prevent it spiraling into civil war, or worse.

But the pro-Moscow separatists — who Kiev and Washington say are backed by Russian special forces — are ignoring the accord’s demands that they disarm and cease occupying buildings in a string of eastern towns.

Russia says Kiev’s leaders — whom it regards as illegitimate — are to blame for the collapse of the accord.

It says ultra-nationalists who were involved in months of Kiev protests that ousted pro-Kremlin president Viktor Yanukovych in February killed rebels in an attack Sunday near the eastern town of Slavyansk.

A funeral for the militants was held on Tuesday. Bells rung loudly from Slavyansk’s Orthodox church and women wept as three coffins were carried out.

Tens of thousands of Russian troops are massed on Ukraine’s eastern border in what NATO believes is a state of readiness to invade.

The United States and NATO have responded by boosting their own forces in eastern Europe.

Biden, after meeting Ukraine’s leaders in Kiev, called on Russia to pull back those forces, and to reverse its annexation last month of Ukraine’s Crimea peninsula.

“We in the United States stand with you and the Ukrainian people,” Biden said in a joint news conference with Ukrainian Prime Minister Arseniy Yatsenyuk.

“There are some who are trying to pull Ukraine apart,” he said, clearly implying Russia.

Washington is threatening to impose more sanctions on Moscow on top of travel bans and asset freezes already applied to members of President Vladimir Putin’s inner circle.

“Time is short to make progress,” Biden warned.

He added that the United States was stepping up to help Ukraine lessen its dependence on Russian gas, fight corruption, and prepare for a May 25 election to choose a new president.

The United States, he said, wants to see Ukraine “hold together as a single state, united and sovereign.”

Yatsenyuk responded that Kiev valued the U.S. support against what he said was a Russia “acting like an armed bandit.”

But in Moscow, Russian Prime Minister Dmitry Medvedev dismissed the US threat of new sanctions.

“I am sure we will be able to minimize their consequences,” he said in a televised speech to the Russian parliament.

However he acknowledged that Russia’s economy was facing an “unprecedented challenge.”

Russia’s finance ministry said Monday the energy-rich nation could tip into “technical recession” over the next three months. Finance Minister Anton Siluanov warned last week that Russia was facing the toughest economic conditions since 2009, when it went into a serious slowdown.

Faced with a worsening crisis, Washington and Moscow have urged each other to rein in each side in Ukraine and revive the Geneva accord.

The European Union, meanwhile, is divided on going further with its own sanctions on Moscow, with some member states worried that increased punishment could jeopardize supplies of Russian gas.

As the crisis plays out, the insurgents in Ukraine’s east remain firmly entrenched in public buildings they have occupied for more than a week.

In the town of Lugansk, close to the Russian border, protesters who have been occupying local security buildings staged a fiery mass demonstration Monday and pledged to hold their own local referendum on autonomy on May 11, the Interfax-Ukraine news agency reported.

Although highly trained military personnel, whose camouflage uniforms are stripped of all insignia, are helping the rebels secure the some 10 towns they hold, Putin denies they are Russian special forces.

But the U.S. State Department released images Monday it claims proves some of the armed “separatists” in Ukraine are actually Russian military or intelligence officers.

In a separate development, Sweden, which is not a NATO member, announced Tuesday it was increasing defense spending because of the “deeply unsettling development in and around Ukraine.” It plans to boost its fleets of fighter jets and submarines.

AFP Photo/Maxim Shipenkov