Several dozen Russian planes including what appeared to be strategic bombers and fighter jets have been spotted in the sky above the Moscow-controlled peninsula of Crimea, witnesses and experts said.

According to Russian media, President Vladimir Putin is poised to visit Crimea on Friday after overseeing the main military parade on Red Square when Russia celebrates its victory over Nazi Germany in World War II.

The number of Jews emigrating to Israel from increasingly volatile Ukraine has jumped by 142 percent this year so far compared to 2013, the Jewish Agency said Sunday.

A total of 762 Jews from Ukraine arrived in Israel in the first four months of the year, compared to 315 in the first quarter of 2013, a spokesman for the quasi-government agency said.

Thousands of pro-Russian protesters assaulted Odessa's police headquarters Sunday, days after deadly clashes and a fire there killed dozens of their comrades in what Kiev charged was a Russian plot to "destroy Ukraine."

The unrest in the southern port city threatened a new front in the Ukrainian government's battle against pro-Moscow militants, with an expanded military operation under way in the east against gunmen holding more than a dozen towns.

Dozens of specialists from the U.S. Central Intelligence Agency and Federal Bureau of Investigation are advising the Ukrainian government, a German newspaper reported Sunday.

Citing unnamed German security sources, Bild am Sonntag said the CIA and FBI agents were helping Kiev end the rebellion in the east of Ukraine and set up a functioning security structure.

Ukraine vowed on Sunday to broaden its operation against pro-Russian rebels as the crisis-hit country observed a second day of mourning after violence that left more than 50 people dead.

National Security and Defense Council chief Andriy Parubiy said the armed forces would expand the "active stage of the operation in other towns where extremists and terrorists are carrying out illegal activities".

Flowers, candles and photos of the dead pile up outside a charred building in the scenic Ukrainian port city of Odessa where anger simmers a day after brutal clashes claimed 42 lives.

Under a warm spring sun, several thousand gathered outside the blackened trade union building, where dozens perished in an inferno that marked the culmination of a day of confrontation.

The head of an OSCE team released by rebels in east Ukraine on Saturday expressed his "deep relief" after an ordeal that lasted more than a week.

"It is happiness, a deep relief," German Colonel Axel Schneider told a small group of journalists on the road outside of the Ukrainian city of Donetsk.

Russian President Vladimir Putin's spokesman said it would be absurd to conduct snap polls in Ukraine amid the raging violence, saying the Kremlin no longer had any influence over rebels in the country's east.

"We do not understand what elections in Kiev they are talking about in European capitals and Washington," Dmitry Peskov was quoted as saying by Russian news agencies.

Seven European OSCE inspectors were freed on Saturday in a flashpoint town in east Ukraine, where surrounded pro-Moscow rebels are battling a fierce military assault amid a soaring national death toll.

The unexpected release was a bolt of good news in Ukraine's startling descent into chaos, after a bloody day in which more than 50 people died -- most of them in a horrific inferno in the southern city of Odessa.

More than 30 people were killed in a "criminal" blaze in Ukraine's southern city of Odessa, as violence spread across the country during the bloodiest day since Kiev's Western-backed government took power.

Ukraine's interior ministry said at least 31 people had died in the fire Friday, with local media reporting that pro-Russian militants were believed to have been in the burning building at the time.