President Barack Obama's approval rating has hit a new low as Americans disapprove of his policies on the economy, health care and the crisis in Ukraine, according to a new poll.

Obama's rating sunk to 41 percent from 46 percent in the first three of the year, according to the Washington Post-ABC News poll released late Monday in a worrying sign for allied Democrats facing legislative elections in November.

Russia warned Tuesday that American astronauts on the International Space Station could be hurt by new U.S.-led sanctions over the escalating crisis in Ukraine, where pro-Moscow militants seized more public buildings in the east.

Washington was resorting to "Iron Curtain" policies from its Cold War-era playbook with the new Western sanctions, which were driving the Ukraine crisis towards "a dead end," Moscow raged.

Moscow on Tuesday vowed to hit back at Japan over its decision to deny visas to 23 Russian nationals as part of additional sanctions linked to the crisis in Ukraine.

The Russian foreign ministry said that Tokyo's decision was "met with disappointment in Moscow, and of course will not be left without a response".

British authorities on Monday launched a money- laundering investigation linked to possible corruption in Ukraine and froze $23 million (17 million euros) in assets.

The announcement came a day before international talks in London aimed at recovering assets which may have been looted under the regime of deposed pro-Moscow president Viktor Yanukovych.

The United States and the European Union on Monday slapped fresh sanctions on Moscow over its role in Ukraine's crisis, as Russia vowed a “painful” response to the measures.

Washington imposed new sanctions on seven Russian officials and 17 firms linked to President Vladimir Putin's inner circle to punish the Kremlin for failing to ease tensions in Ukraine.

The pro-Russian mayor of the town of Kharkiv in eastern Ukraine was in critical condition on Monday after he was shot while riding his bicycle, local officials said.

Gennady Kernes was "hit by a bullet in the back" during an attack by an unidentified gunman, a town hall official said. "He is being operated on. Doctors are fighting for his life."

Britain and France deployed eight fighter jets on Monday to reinforce NATO air patrols over the Baltics as tensions rise with Russia over Ukraine, officials said.

Four British Typhoon jets arrived in Lithuania to start their mission while four French Rafale jets touched down in Malbork, northeast Poland, their defense ministries said.

Heavily armed pro-Russian gunmen on Monday seized another town in east Ukraine, storming the town hall in Kostyantynivka and setting up barricades, as militants attacked a bank owned by an anti-Moscow billionaire.

Kostyantynivka has 80,000 inhabitants and is located mid-way between the flashpoint town of Slavyansk and the regional hub city of Donetsk, both of which are also under the control of insurgents.

France on Sunday warned of "incalculable consequences" if the situation in Ukraine deteriorates, calling on Russia and on pro-Russian rebels in the former Soviet republic to de-escalate the crisis.

"The situation is very worrying," Foreign Minister Laurent Fabius said on French television. 

Former Russian tycoon Mikhail Khodorkovsky was heckled on Sunday by angry pro-Moscow activists in the eastern Ukrainian city of Donetsk and barred from entering the rebel-held city hall.

Khodorkovsky, who spent a decade in prison in Russia and now lives in self-imposed exile in Switzerland, is visiting eastern Ukraine to "see for himself what's happening there," his spokeswoman Olga Pispanen told Agence France Presse.