Hungary's foreign minister, Peter Szijjarto, will travel to Moscow on Wednesday to meet his Russian counterpart Sergei Lavrov, his office told AFP.

"They will discuss bilateral issues and exchange views on the latest international developments," the foreign ministry said in an email.

Ukraine's prime minister on Tuesday called for fresh talks with Russia on "neutral territory" as deadly fighting rumbled on in the country's east.

"We invite the Russian Federation to hold serious negotiations on a neutral territory. The US and EU are helping us with this," Arseniy Yatsenyuk was quoted as saying by the Interfax Ukraine news agency.

NATO warned on Tuesday of a "very serious" build-up of Russian soldiers and weapons inside Ukraine and on the border, as Germany's foreign minister urged Kiev and Moscow to respect a tattered peace plan.

The West is keeping up intense pressure on Russia over Ukraine following a bad-tempered G20 summit in Australia at the weekend which Russian President Vladimir Putin left early.

Russia said on Tuesday it hopes its ties with the EU had not yet crossed the point of no return over the Ukraine crisis after Brussels slapped more sanctions against Kremlin-backed separatists.

"We are hoping that the 'point of no return' has not yet been crossed," Russian Foreign Minister Sergei Lavrov was quoted as saying by the state news agency TASS at a meeting in the Belarussian capital Minsk.

The EU must speak with one voice to Russia over Ukraine and be ready to send a "clear message" it will impose more sanctions if necessary, Ukrainian Foreign Minister Pavlo Klimkin told AFP in an interview Monday.

"Now it is time to have a clear message to Moscow that further destabilization in Ukraine will trigger further steps by the EU," Klimkin said as he visited Brussels for talks with new EU foreign affairs chief Federica Mogherini.

President Vladimir Putin stuck to his guns as he refused to say where Moscow-backed Ukrainian separatists receive heavy arms from and said that people fighting a just cause "will always get weapons".

"Where did they get the armoured vehicles and the artillery systems?" Putin said in reply to a question from German TV network ARD in an interview broadcast Sunday.

Russia expelled several Polish diplomats for spying on Monday, deepening the worst East-West crisis since the Cold War as the EU eyed fresh sanctions against Moscow over the violence in Ukraine.

Fresh bloodshed in Ukraine between pro-Kremlin rebels and Kiev's forces added to the tensions after Russian President Vladimir Putin left a G20 summit in Brisbane early amid criticism from western leaders.

Ukrainian President Petro Poroshenko claimed his country was "prepared for total war" as fighting continued Sunday around the rebel stronghold of Donetsk.

After a week in which Kiev said several unmarked armored convoys of troops crossed the Russian border to reinforce rebels in the east, Poroshenko toughened his rhetoric, telling the German daily Bild: "I am not afraid of a war with Russian troops."

A former member of the Latvian parliament expressed "surprise" Sunday after Russian media claimed he had been expelled from Russia for spying, in another Cold War-style incident in the ex-Soviet Baltic states.

Aleksejs Holostovs, a former member of the Harmony political party, which draws much of its support from the country's large Russian minority, told the LETA news service he was "surprised" at the allegation of spying from Russia. 

The United States Embassy in Lebanon denied on Sunday a report saying that Washington had filed an official complaint to the Lebanese government for attempting to strike a deal with Russia on supplying the army with used T-72 tanks.

The embassy stressed in comments to Naharnet that “the report is false.”