Pro-Russian rebels fighting Ukraine's Western-backed government have launched their most serious attacks yet on Lugansk International Airport in the east of the country, a military source said Sunday.

The assaults, which took place Saturday evening and Sunday morning in the immediate wake of the inauguration of Ukraine's new President Petro Poroshenko, did not result in any injuries among the defenders, he said.

Germany's foreign minister said Sunday that only "substantial" talks between Moscow and Kiev would help defuse the Ukraine crisis, ahead of an upcoming meeting with his Russian counterpart.

Frank-Walter Steinmeier told a German Sunday newspaper that a resolution was still far off but several developments had given rise to hope, such as the Ukrainian presidential vote, or the first meeting between the Russian and Ukrainian presidents.

Ukraine President Petro Poroshenko gets down to work Sunday, after vowing at his inauguration to avert civil war and mend ties with Russia as his nation faces disintegration and economic collapse.

Poroshenko took the oath of office on Saturday, the day after holding his first meeting with Russian President Vladimir Putin since a May 25 election victory entrusted him with stemming a bloody crisis that has shaken the post-Cold War order and redrawn Europe's map.

U.S. Vice President Joe Biden pledged new assistance Saturday to ex-Soviet republics Georgia and Moldova aimed at helping them stand up to a resurgent Kremlin and establish closer European ties.

Biden held separate meetings with Georgian President Georgy Margvelashvili and his Moldovan counterpart Nicolae Timofti on the sidelines of Western-backed tycoon Petro Poroshenko's inauguration as Ukraine's head of state.

U.S. Secretary of State John Kerry Saturday voiced hope that there would be a breakthrough on ending the Ukraine crisis, enabling Washington to avoid imposing new sanctions against Russia. 

"I hope that in the next few days we can see some steps taken that will reduce the tensions ...I'm confident there are ways forward, we look for Russia’s help and our hope is that we won’t have to move to more serious sanctions and other steps," he said.

Direct bilateral talks between Iran and the United States will be "a timely opportunity" to try to advance a nuclear deal with world powers, a U.S. official told Agence France Presse on Saturday.

"These consultations come at an important juncture of the negotiations," the senior administration official said just after both sides announced that surprise talks would take place next week in Geneva.

Russian President Vladimir Putin on Saturday ordered the border service to reinforce the state border with Ukraine, the Kremlin press service told Russian news agencies.

Putin issued the order for border guards to take all necessary measures to prevent illegal crossings, Russian news agencies reported.

Russia demanded the release of two journalists on Saturday that were detained by Ukrainian security forces at a checkpoint in the restive east of the country.

"We demand that Kiev take immediate measures to free these Russian citizens without conditions," the Russian foreign ministry said in a statement.

Russia and Ukraine appeared to have made a long-awaited breakthrough in efforts to resolve a damaging crisis in their relations after conciliatory talks on the sidelines of Friday's D-Day anniversary ceremonies.

Vladimir Putin and Ukrainian president-elect Petro Poroshenko embraced the spirit of the day that signaled the end of World War II by announcing they would jointly seek a ceasefire in the conflict between government forces and pro-Russian rebels in southeastern Ukraine.

A Ukrainian police officer was killed and two others injured Friday when insurgents near the separatist stronghold of Slavyansk launched a mortar attack on a checkpoint, officials said.

The insurgents, who had taken up position in a church, opened fire on Ukrainian forces, deputy interior minister Serhiy Yarovy said in a statement.