On Thursday, John Kerry said that Russia had refused to take "a single concrete step" to implement last week's Geneva agreement aiming to defuse tensions in eastern Ukraine, saying Moscow had instead put its faith in "distraction, deception and destabilization."

Speaking in Washington, Kerry said that Ukraine's interim authorities were making concessions as a result of the Geneva talks - listing pledges announced by interim Prime Minister Arseniy Yatsenyuk. These included promises of "comprehensive constitutional reform that will strengthen the powers of the regions," and to protect the status of the Russian language.

Yatsenyuk had, Kerry said, "directly addressed the concerns of the Russians - and he did so on day one."

The top US diplomat also hinted at further sanctions, a day after President Barack Obama had said subsequent steps were "teed up," in case the situation deteriorated. He said it would be both a "grave" and "expensive mistake" for Russia to continue on its current path.

"The window to change course is closing," Kerry said. "President Putin and Russia face a choice. If Russia chooses the path of de-escalation … all of us will welcome it. But, if Russia does not, the world will make sure that the costs for Russia will only grow."

Military maneuvers near border

Kerry was speaking after Russia had announced the resumption of military maneuvers near the border. This followed Ukraine's Interior Ministry reporting that its troops had killed up to five militants in the east of the country.

"We are compelled to react to such a situation," Russian Defense Minister Sergei Shoigu was reported as saying by domestic news agencies. "From today, military exercises have started in regions bordering on Ukraine involving battalions of tactical forces of the southern and western military districts."

Ukraine's military earlier in the week restarted operations seeking to reclaim control of government buildings in several eastern towns held by pro-Russian separatists. This prompted a warning from Russian President Vladimir Putin that military action in the east of the country would "have consequences."

UN Secretary-General Ban Ki-moon warned that the tensions were threatening to "spin out of control," urging all sides to "refrain from violence."

Germany halting arms exports

On Thursday, the state broadcaster ARD and the daily Süddeutsche Zeitung reported that the German government, responding to a formal inquiry from the opposition Greens, would temporarily stop approving any military exports to Russia. All arms exports by German companies require approval from a special government panel, but not from parliament. Both publications quoted the government's response as saying that "because of the current political situation, no permission for the export of military products to Russia is being granted."

The reports also quoted the Economy Ministry as announcing that it had "begun an investigation into what can be done about export approvals that have already been granted."

In Berlin on Friday, Chancellor Angela Merkel will meet her Polish counterpart, Donald Tusk, a vocal advocate of a tough stance against Russia. Poland, a NATO and EU member, borders Ukraine.

msh/jm (AFP, AP, dpa, Reuters)