Rebel strongholds in eastern Ukraine braced for more fighting on Thursday as Western leaders piled pressure on Kiev to strike a truce with pro-Russian separatists.

French President Francois Hollande and German Chancellor Angela Merkel in a telephone call urged the Western-backed President Petro Poroshenko to move toward a political solution and said he promised to exercise "restraint" in the continuing drive to establish control in eastern Ukraine.

The United States is prepared to impose tough new economic sanctions on Russia "very soon" if Moscow refuses to sever ties with Kremlin-backed separatists in eastern Ukraine, a U.S. official told lawmakers Wednesday.

But senators, frustrated with U.S. inaction, said Washington's repeated threats ring hollow.

Russia said Wednesday it had arrested a Ukrainian helicopter navigator and charged her over the deaths of two Russian journalists in an attack in eastern Ukraine last month, prompting a furious reaction from Kiev.

Russian investigators said Nadiya Savchenko, one of the few women serving in the Ukrainian airforce, was an accomplice in what they claimed was the deliberate murder of the journalists.

Rebel strongholds in eastern Ukraine braced for more fighting on Wednesday as European leaders prepared to pile new pressure on President Petro Poroshenko for a truce with pro-Russian separatists.

French President Francois Hollande and German Chancellor Angela Merkel were expected to push the Western-backed leader on a ceasefire in three-way telephone talks but Kiev has until now shrugged off calls to halt an offensive that has reclaimed a string of key rebel towns.

Russia said Wednesday it had approved the candidacy of the former U.S. ambassador to Kiev as Washington's new envoy to Moscow, at a time of sharply heightened tensions over Ukraine.

If he is approved by the U.S. Senate, veteran diplomat John Tefft -- known for backing the pro-Western aspirations of former Soviet states -- will succeed Michael McFaul, who abruptly quit his post in February after just two years on the job.

As a Ukrainian government offensive sends separatists retreating from their strongholds in the country's restive east, there are signs that Moscow is seeking to distance itself from the pro-Russian rebels.

Facing the threat of biting Western sanctions that could further shake Russia's teetering economy, President Vladimir Putin has watched a string of rebel defeats without taking any action -- drawing accusations from separatist sympathizers at home that he is betraying their cause.

Ukraine sought on Tuesday to ban the Communist Party more than two decades after the Soviet Union's collapse because of its alleged support for pro-Russian insurgents battling government forces in the separatist east.

The party immediately denounced Justice Minister Pavlo Petrenko's action with the Kiev District Court as an attempt to curb Ukrainians' political rights.

Russian Senate speaker Valentina Matviyenko said on Tuesday she abhorred Kiev's "scorched earth" tactics in eastern Ukraine, but that the Kremlin was unlikely to seek a new mandate for military intervention in the crisis.

"What is happening now is a scorched earth tactic, a purge of the territory in certain areas of eastern Ukraine that the National Guard has occupied," said Matviyenko, the speaker of the Federation Council and formally Russia's third most senior politician.

Ukraine on Tuesday brushed off strong European pressure as it rejected talks with pro-Russian rebels on a truce to halt a bloody insurgency convulsing the ex-Soviet nation until they laid down their arms.

The unconditional stance reflected a new confidence in Kiev that it was on the verge of quashing a rebellion it views as Moscow's retribution for the February ouster of a Kremlin-backed leader and the decision to pursue a historic alliance with the West.

NATO is drawing up plans to ensure its members can respond more quickly to crises in the aftermath of Russia's "aggression" in Ukraine, the alliance's chief said Monday.

Secretary General Anders Fogh Rasmussen said the transatlantic alliance would review a proposed readiness "action plan" at an upcoming summit in September in Britain and also predicted European members were poised to reverse a long decline in military spending dating back to the Cold War.