The West should impose tougher sanctions on Russia, which is waging a "hidden war" in eastern Ukraine, Ukraine's acting Foreign Minister Andriy Deshchytsia said in an interview with German newspaper Die Welt.
The United States and the European Union have already imposed several rounds of sanctions on Russian individuals and some companies, accusing Russian President Vladimir Putin of stoking the eastern rebellions, a charge Putin has denied.
"The West should impose more severe sanctions that hit specific economic sectors such as banking and target Russian decision makers," he said in an advance copy of an interview due to be published in Saturday's print edition.
"It's also important to impose preventative sanctions before Russia does even more damage," he said.
Deshchytsia said German politicians should be aware "that Russia will not stop at Luhansk and Donetsk" and added that if they do not want to be held responsible for destabilising the region, the German government has to be tougher toward Russia.
Germany has close economic ties to Russia and German industry is ramping up efforts to dissuade Chancellor Angela Merkel from imposing tough new economic sanctions on Russia over Ukraine, warning of lasting damage to domestic firms and the broader economy if Moscow is hit hard.
Deshchytsia said Putin was not planning to "openly invade" Ukraine because he feared consequences such as further sanctions and the Kremlin was therefore sending "terrorists to create unrest and bring the eastern part of the country under his control."
He said most of the weapons being used by "rebels" had come from Russia such as their machine guns, which he said came from Russian army stocks, were used exclusively by the Russian military and could not be acquired elsewhere.
"Border guards arrested smugglers who wanted to bring these weapons intoUkraine," he said.
"In addition, we've intercepted numerous telephone conversations between separatist leaders and Russian agents which prove that they are getting their orders from Russia."
Deshchytsia said he was worried that citizens in some places in eastern Ukraine would be prevented from voting in the presidential election set for May 25 because they would be intimidated by "terrorists" and fear reprisals.
He said Ukraine was negotiating with Russia about the price of gas and was prepared to pay the market price of around $300 per 1,000 cubic meters.
He added that Ukraine was also negotiating with Poland, Slovakia and Hungary about buying gas and was working on energy efficiency projects to become less dependent on Russian state-controlled gas producer Gazprom.
Russia has warned that it will not supply Ukraine with gas in June unless Kiev pays in advance by June 2, raising fears that deliveries to Europe could be affected.
(Reporting by Michelle Martin; Editing by Lisa Shumaker)