Speaking in the Philippines, US President Barack Obama said the sanctions would include export restrictions on high-tech defense goods in a bid to ratchet up pressure on Russian President Vladimir Putin, blamed for the worst East-West crisis since the Cold War.

Top EU officials were also meeting in Brussels to step up European sanctions on Russia as part of a coordinated global effort against Moscow.

On the ground in eastern Ukraine, tensions spiked further as gunmen stormed the town hall and police offices in Kostyantynivka, the latest in a string of insurgent assaults on towns in the region.

The pro-Russian mayor of Kharkiv was also left fighting for his life after unidentified gunmen shot him in the back.

In the flashpoint town of Slavyansk, rebels refused to release a group of international monitors from the OSCE after presenting them to the media as "prisoners of war" in what Germany said was a "repugnant" display.

Meanwhile, the threat of a full-scale invasion loomed large over the ex-Soviet country, with tens of thousands of Russian troops massed on the border and Ukraine's prime minister warning of efforts to start a "third world war".

Obama also said Washington would unveil a list of "individuals and companies" that will be sanctioned to build pressure on Putin and Russia's recession-hit economy.

The Group of Seven (G7) top economies has also vowed swift joint action and EU diplomats were expected to expand the bloc's sanctions against Moscow at a meeting later on Monday.

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