May 12 (Reuters) - Australia's Viva Energy on Thursday posted a 65% jump in operating earnings for the first four months of the year, as strong demand for refined products, tight supplies due to sanctions on Russia, and lower exports from China aided margins.

Global energy supply has been upended since Russia's invasion of Ukraine in late-February prompted the West to impose sanctions on Moscow, while China's hard-line zero-COVID strategy has led to extended curbs that have compounded supply woes.

"We have seen unprecedented movements in refining margins over this past month due to recent geo-political developments and a broader decline in global refining capacity and production," Chief Executive Officer Scott Wyatt said.

Margins at Viva's Geelong refinery in Victoria more than doubled to $26.4 per barrel in April from March, and were much higher than an average of $8.3 per barrel in the March quarter, Viva said.

The company's un-audited earnings before interest, tax, depreciation and amortization (EBITDA) on a replacement cost basis were A$308 million ($213.8 million) for the four months to April.

It warned, however, that oil markets remained extremely volatile, and higher shipping expenses and rising energy costs would provide some longer-term headwinds. ($1 = 1.4407 Australian dollars) (Reporting by Navya Mittal in Bengaluru; Editing by Anil D'Silva and Uttaresh.V)