A pro-Russian man in military fatigues poses for a photo with a child on Monday as he stands guard outside a regional administration building seized in the night by separatists in Kostyantynivka. Photo: AFP

The United States on Monday 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.

Washington is also tightening licensing requirements for certain high-tech exports to Russia that could have a military use, the White House announced in a statement issued in Manila, where President Barack Obama is on a state visit.

The White House said the third round of sanctions was prompted by Moscow's failure to adhere to an April 17 agreement in Geneva.

The seven Russians sanctioned include two Putin allies: Igor Sechin, head of Russia's major oil company Rosneft, and Sergei Chemezov, head of Rostec, a Russian state-owned high-tech products company.

The sanctions follow up on previously announced measures targeting other members of Putin's political and inner circle and a Russian bank.

The White House said those measures would only be imposed by the US and its European allies if Russia actually invades Ukraine.

A senior US official said the sanctions would be rolled out in coordination with new EU sanctions, and that existing measures were already having a "substantial impact" on the Russian economy.

Diplomatic sources in Brussels meanwhile said that a further 15 officials would be subject to the same visa bans and asset freezes already imposed on 50 others.

The sources said the action would be taken because Moscow shows no sign of reversing course in Ukraine.

Russian Deputy Foreign Minister Sergei Ryabkov told the Interfax news agency that his country would respond to the sanctions.

"We are certain that this response will have a painful effect on Washington," he said.

The companies sanctioned include Avia Group limited, the Severny Morsoy Bank, InvestCapital Bank, Transoil, Volga Resources Group, SMP Bank, Sobin Bank and Zest Leasing.

China's foreign ministry on Monday restated its opposition to placing sanctions on Russia over the crisis in Ukraine, after leaders of the Group of Seven major economies agreed to swiftly impose further punitive measures.

In eastern Ukraine on Monday, pro-Moscow rebels showed no sign of curbing their uprising, seizing public buildings in another town in the east, Kostyantynivka.

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

Germany on Monday 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.