AMERICA has frozen assets and imposed visa bans on seven powerful Russians close to President Vladimir Putin and also sanctioned 17 companies in reprisal for Moscow's actions in Ukraine.

President Barack Obama said the moves, which add to measures taken when Russia annexed Crimea last month, were to stop Mr Putin fomenting rebellion in eastern Ukraine. Mr Obama added he was holding broader measures against Russia's economy "in reserve".

Among those sanctioned were Igor Sechin, head of state energy firm Rosneft and a close lieutenant of Mr Putin for more than two decades, and Deputy Prime Minister Dmitry Kozak. A Russian deputy foreign minister was quoted as expressing "disgust" at the White House announcement.

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The European Union, with more to lose than Washington from sanctions against Russia, which is a major energy supplier and trading partner for the EU, is also imposing more sanctions.

Travel bans and asset freezes will be imposed by Brussels on a further 15 Russian officials as the EU continues to ramp up its response over the crisis in Ukraine.

Foreign Secretary William Hague, who will travel to Ukraine, Georgia and Moldova next week, said the "doors of diplomacy remain fully open" but warned Moscow it would pay an even "heavier price" if it fails to act to quell the unrest.

America's moves will deny export licences for any high-technology items that could contribute to Russian military capabilities and will revoke any existing export licences that meet these conditions, the White House said.

It was the third round of sanctions the United States has imposed over Crimea and Russia's troops build-up on the border. All the sanctions have been aimed at individuals and businesses.

American officials had said the new list would include Putin's "cronies" in the hope of changing his behaviour.

Mr Obama said: "The goal is not to go after Mr Putin personally. The goal is to change his calculus with respect to how the current actions he is engaging in in Ukraine could have an adverse impact on the Russian economy over the long haul.

"To encourage him to actually walk the walk and not just talk the talk when it comes to diplomatically resolving the crisis in Ukraine."

However, several US Republican lawmakers said the latest sanctions were too mild to deter Moscow from further action in Ukraine and demanded broader measures.

Tennessee Senator Bob Corker, the top Republican on the Senate Foreign Relations Committee, called the new sanctions "just a slap on the wrist".

Moscow insists a rebellion among Russian-speakers in the east against the Kiev authorities that took power after the overthrow of a Kremlin-backed president in February is a home-grown response to a coup and denies having forces on the ground.

In eastern Ukraine, pro-Moscow rebels showed no sign of curbing their uprising, seizing the police headquarters and the municipal administration building in Kostyantynivka, an industrial city in the eastern Donetsk region. Separatists in the province have proclaimed an independent "People's Republic of Donetsk".

The high-profile mayor of another eastern city Kharkiv, Ukraine's second biggest, was fighting for life after being shot while out cycling.

Gennady Kernes, 54, the pro-Russian mayor of Kharkiv, was shot in the back, probably by someone hidden in nearby woods, said Iryna Kushchenko, spokeswoman for the local government. Mr Kernes' bodyguards were following in a car but were not close enough to intervene.

Meanwhile, Germany has demanded Russia act to help secure the release of seven unarmed European military monitors, including four Germans, who have been held by the rebels since Friday.