KIEV, April 27 - Pro-Russian rebels paraded European monitors they are holding in eastern Ukraine on Sunday, freeing one but saying they had no plans to release other seven as the United States and Europe prepared new sanctions against Moscow, Reuters reported.

U.S. President Barack Obama called for the United States and Europe to join forces to impose stronger measures to restrain Moscow.

The White House said it will add names on Monday of people close to President Vladimir Putin and firms they control to a list of Russians hit by sanctions over Ukraine, and also impose new restrictions on high tech exports.

The European Union is expected to follow suit by adding to its own list of targeted Russian people and firms, but Washington and Brussels have yet to reach agreement on wider measures designed to hurt the Russian economy more broadly.

In Donetsk, where pro-Russian rebels have proclaimed an independent "people's republic,” armed fighters seized the headquarters of regional television and ordered it to start broadcasting a Russian state TV channel.

After Ukrainians overthrew a pro-Russian president, Putin overturned decades of international diplomacy last month by announcing the right to use military force on his neighbor's territory. He has seized and annexed Ukraine's Crimea peninsula and massed tens of thousands of troops on the frontier.

Heavily armed pro-Russian gunmen have seized buildings in towns and cities across eastern Ukraine. Kiev and its Western allies say the uprising is directed by Russian agents. Moscow denies it is involved and says the uprising is a spontaneous reaction to oppression of Russian speakers by Kiev.

An international agreement reached this month calls on rebels to vacate occupied buildings, but Obama said Russia had not "lifted a finger" to push its allies to comply.

"In fact, there's strong evidence that they've been encouraging the activities in eastern and southern Ukraine."

The Organization for Security and Cooperation in Europe has sent unarmed monitors to try to encourage compliance with the peace deal. The pro-Russian rebels seized eight European monitors three days ago and have been holding them at their most heavily-fortified redoubt in the town of Slavyansk.

One, a Swede, was permitted to leave on Sunday after OSCE negotiators arrived to discuss their release. A separatist spokeswoman said the prisoner had been let go on medical grounds, but there were no plans to free the rest.

The captives, from Germany, the Czech Republic, Denmark, Poland and Sweden, were paraded before reporters on Sunday and said they were in good health.

The OSCE, a European security body, includes Russia. Its main Ukraine mission was approved by Moscow, although the Europeans held in Slavyansk were on a separate OSCE-authorized mission that did not require Russia's consent.

Vyacheslav Ponomaryov, the rebel leader who has declared himself mayor of Slavyansk, has described them as prisoners of war and said the separatists were prepared to exchange them for fellow rebels in Ukrainian custody.

Ponomaryov also said his men had captured three officers with Ukraine's state security service who, he said, had been mounting an operation against separatists in the nearby town of Horlivka.

The Russian television station Rossiya 24 showed footage it said was of a colonel, a major and a captain. They were shown seated, with their hands behind their backs, blindfolded, and wearing no trousers. At least two had bruises on their faces.

Ukraine's State Security Service said the three had been part of a unit which went to Horlivka to arrest a suspect in the murder of Volodymyr Rybak, a pro-Kiev councilor whose body was found last week in a river near Slavyansk. (rt/ez)