Ukraine government forces on Saturday accused pro-Russian separatists of using Grad multiple rocket launchers overnight in the country's war-torn east in violation of a truce deal signed last month.

Military spokesman Andriy Lysenko told reporters that separatists fired the heavy weapons towards the village of Novotoshkivka from their Lugansk stronghold.

Vandals Saturday destroyed an improvised memorial created by supporters of Russian opposition leader Boris Nemtsov on the bridge where he was shot a month ago, the Echo of Moscow radio station reported Saturday.

Supporters had regularly refreshed flowers and candles on the bridge next to the Kremlin where the 55-year-old former deputy prime minister and prominent critic of President Vladimir Putin was gunned down on February 27.

A default by Ukraine on $3 billion in debt to Russia this year could threaten the International Monetary Fund's lifeline to the embattled country, an IMF spokesman said Thursday.

The Ukrainian government has begun negotiations with creditors for $15 billion in debt relief, part of a $40 billion, four-year financial rescue envisioned by the IMF.

Russian President Vladimir Putin and his Iranian counterpart Hassan Rouhani voiced hope Thursday for progress at crunch talks over Iran's nuclear drive, the Kremlin said. 

"Hope was expressed for success at the new round of talks in Lausanne," the Kremlin said in a statement after Rouhani called Putin to discuss the negotiations in Switzerland.

The United States fears that Moscow and pro-Russian separatists will reignite the armed conflict in eastern Ukraine following a February ceasefire that has largely held, a U.S. diplomat warned Wednesday.

"Russia can reignite the conflict at any time of its choosing," the senior State Department official told journalists.

A Moscow-friendly Greece could paralyze NATO's ability to react to Russian aggression, former U.S. national security adviser Zbigniew Brzezinski warned Wednesday.

Athens could use its veto to slow the alliance's response if Russia set its sights on its Baltic member states, he told Poland's Dziennik Gazeta Prawna.

Russia and Cuba worked on their joint strategy against the U.S. economic embargo on the communist island Tuesday, during a visit by Moscow's Foreign Minister Sergei Lavrov.

Lavrov, the first Russian cabinet member to visit Cuba since Washington and Havana announced they would set aside their Cold War enmity and renew diplomatic ties, met President Raul Castro at the start of a four-country Latin American tour.

NATO jets were scrambled Tuesday to escort Russian fighters and nuclear-capable bombers flying near the Baltic states and Sweden with their transponders switched off, sparking protests over the danger they posed to civil aviation.

Lithuania's defense ministry spokeswoman Asta Galdikaite said NATO air policing aircraft identified two Tu-22 type bombers and two SU-27 jets. The Swedish military also confirmed the aircraft showed up on their ground radar.

Russia said Tuesday it had issued a fresh broadcast license to CNN after the U.S. news channel went off air last year, blaming restrictions on foreign-owned media.

A spokeswoman for Russia's media watchdog Roskomnadzor confirmed an order was given to issue CNN with a license to broadcast nationwide for 10 years.

Russia on Tuesday warned the United States against sending a ballistic missile defence system to South Korea, saying it could threaten regional security.   

Washington says it wants to deploy the system, known as THAAD, to South Korea as a deterrent to military provocation by North Korea.