BERLIN--The sharp devaluation of the ruble is cause for concern in Europe but won't lead to an early review of the European Union's economic sanctions against Moscow, German officials said on Wednesday.

The assessment came after German Chancellor Angela Merkel, Russian President Vladimir Putin, Ukraine President Petro Poroshenko and French President François Hollande discussed the situation in Ukraine in a telephone conference late Tuesday evening, the first such exchange in weeks.

A German official said Wednesday the rapid economic deterioration in Russia was causing "serious concern" in Berlin, adding, "we are worried both about the economic and the security implications."

Yet this was no ground to bring forward the expected review of the European Union's economic sanctions against Russia, which are due for renewal in July, the official added.

"The question of the sanctions is separate," the official said.

While Berlin is keeping a close eye on developments in the Russian economy, it sees no direct relation between the sharp fall in the ruble and the sanctions, Steffen Seibert, Ms. Merkel's spokesman told journalists on Wednesday.

The currency crisis has "a large number of causes", most of which predate the Ukraine crisis and "have nothing to do with the sanctions," Mr. Seibert said.

Last month, German Foreign Minister Frank-Walter Steinmeier told an economic conference in Berlin that a collapse of the Russian economy wouldn't be in the West's interest and wouldn't help improve the security situation in Europe.

The EU sees subtle signs of Russia becoming more cooperative on the Ukrainian crisis, said Federica Mogherini, the bloc's foreign-policy chief, on Wednesday, adding that "consequent choices and acts" were needed to support the statements.

And a softer rhetoric can be heard from Moscow in the interview of Russian Foreign Minister Sergei Lavrov to French TV channel France24 on Monday, who called Ukrainian President Petro Poroshenko "the best chance" for the country and reassured that Russia won't annex eastern territories in the country's east as it did with Crimea in March.

"What counts is not the expression of willingness--[it] is the real willingness and real...delivery on the Minsk agreements," Ms. Mogherini said in Kiev. More than a thousand people have been killed in the conflict zone in Ukraine's east since the government forces and Russia-backed separatists signed a peace accord in the Belarusian capital of Minsk three months ago.

In their joint conference call, the German, French, Russian and Ukrainian leaders agreed that the sides have to convene again this week to cement the implementation of the agreement, as the fighting decreased significantly in observance of the "day of silence" since last Tuesday. The intensity of isolated shootings declined, but not completely, as Kiev reported one servicemen wounded in the last 24 hours.

This week's meeting of the contact group of Russia, Ukraine and the Organization for Security and Cooperation in Europe is expected to focus on setting the line separating government-controlled areas from separatist-occupied territories and agreeing the terms of a comprehensive prisoner exchange.

With winter approaching, the leaders agreed humanitarian aid must be strengthened. In particular, Ukraine must be allowed to bring its own relief supplies safely even in the eastern parts of the country that is controlled by separatists, the Chancellery's statement said.

Little signs of relief can be seen in the Ukrainian capital, however.

A full-scale war may be detonated by Russia anytime, the newly appointed Ukrainian security and defense secretary Oleksandr Turchynov warned in a briefing Wednesday. "Our war will finish only after we liberate all of the Ukrainian territory, including the Autonomous Republic of Crimea," said Mr. Turchynov, the former Parliamentary Speaker, as he called for the strengthening of the country's defense and reserves buildup.

Speaking in Polish parliament on Wednesday, Ukraine's President Poroshenko said he was committed to the NATO course for Ukraine and promised to submit a bill renouncing his country's nonaligned status upon returning to Kiev.

Stefan Lange in Berlin and Ulrike Dauer in Frankfurt contributed to this article.

Write to Bertrand Benoit at bertrand.benoit@wsj.com and Gregory L. White at greg.white@wsj.com

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