BRUSSELS: Europe and the United States ramped up pressure Thursday on Russia over the annexation of Crimea, imposing sanctions on senior figures close to President Vladimir Putin, who retaliated in a deepening of the worst East-West crisis since the Cold War.

The EU move came on the heels of an announcement by US President Barack Obama of a new round of punitive measures against 20 Russian lawmakers and senior government officials, in addition to 11 individuals already targeted.

Obama, who threatened to target the broader Russian economy if Moscow did not reverse course, said: "Russia must know that further escalation will only isolate it further from the international community."

Among those named were top businessmen close to President Vladimir Putin such as billionaires Gennady Timchenko, Arkady Rotenberg and Boris Rotenberg plus a bank used by close associates.

Russia 'cannot continue'

"Russia must understand that it cannot continue, that it must take the path of dialogue," French President Francois Hollande said as he announced the EU measures.

The 28-nation bloc also cancelled an EU-Russia summit planned for June and called for the dispatch of international or EU monitors to Ukraine.

In Moscow, where the lower house of parliament rubber-stamped the absorption of the rebel peninsula, authorities issued their own list of sanctions against nine US officials, including ranking political figures and presidential aides.

In a tit-for-tat measure, Moscow listed Obama aides Caroline Atkinson, Daniel Pfeiffer and Benjamin Rhodes and senators Mary Landrieu, John McCain and Daniel Coats.

"There should be no doubt: each hostile attack will be met in an adequate manner," the Russian foreign ministry said.

There was no immediate Russian response to the EU sanctions. The European Union did not immediately identify the 12 individuals added to its sanctions list, saying they would be named on Friday.

European Council president Herman Van Rompuy said "some are really highly-placed".

Van Rompuy added: "Sanctions are not a question of retaliation... not a goal in themselves but a means to an end.

"Our goal is to restore Ukraine's sovereignty."

Europe's leaders stopped short of wider economic sanctions but insisted they were ready to ratchet up the pressure in case of further signs of Russian aggression.

"If there's any further destabilisation in Ukraine, then there should be further wide ranging measures taken," said British Prime Minister David Cameron.

But some EU nations are heavily dependent on Russian oil and gas. Consequently, the bloc is divided on how far it should go, with many members reluctant to raise the economic stakes.

Ukraine getting EU support

In a fresh sign of support for Ukraine's transitional authorities, EU leaders on Friday will sign with interim premier Arseniy Yatsenyuk the political sections of a broad EU Association Agreement. Its rejection back in November by Ukraine's pro-Kremlin president Viktor Yanukovych sparked the protests that led to his fall.

EU efforts to diversify energy supplies after Russia cut deliveries to Ukraine in 2006 and 2009 have so far failed to make much headway and the EU leaders are due to discuss the issue again.

The International Monetary Fund said meanwhile it made "significant progress" in talks with Ukraine's new Western-backed government over the resumption of a vital support programme for the crisis-hit state.

Standard & Poor's on the other hand revised the outlook on Russia's credit rating to negative from stable on rising political and geopolitical risks linked to the Ukraine crisis and accompanying Western sanctions.

Sanctions and calls for talks have so far done nothing to halt Russian military advances, with Kiev's new Western-backed government preparing a Crimean evacuation plan for thousands of Ukrainian soldiers and their families.

The march by Moscow's troops and pro-Kremlin militias across the mostly Russian-speaking region roughly the size of Belgium has been unhalting since the day Putin won parliamentary approval to use force against his ex-Soviet neighbour following the February 22 fall of Yanukovych.

Kiev's untested leaders have voiced fears that President Vladimir Putin has set his sights on Russified southeastern swathes of Ukraine as part of his self-declared campaign to "protect" compatriots.

Yatsenyuk said Ukraine would "respond firmly, including through military means," if Russia attempts to annex the country's mainly Russian-speaking regions.

But the Pentagon said Russian Defence Minister Sergei Shoigu promised his US counterpart Chuck Hagel in a telephone call Thursday that Moscow would not assault eastern Ukraine.

Hagel voiced concern about Russian military movements but Shoigu assured him that "the troops he has arrayed along the border are there to conduct exercises only and they have no intention of crossing the border into Ukraine and that they would take no aggressive action," Pentagon spokesman Rear Admiral John Kirby told reporters.

But Russia's Federal Customs Service announced tougher and more time-consuming inspections of goods entering the country from Ukraine that it said were aimed at intercepting possible illicit shipments of arms.