By Matthias Williams

KIEV (Reuters) - A senior Ukrainian diplomat was headed to the Russian border on Thursday morning to help the sister of jailed Ukrainian pilot Nadezhda Savchenko after Kiev accused Russian border guards of seizing her passport and preventing her from leaving Russia.

Vira Savchenko was in a Ukrainian diplomatic car trying to cross back into Ukraine when she was stopped at the Russian border and guards seized her passport, Ukrainian officials said.

Savchenko had been told by a Russian border guard that she was on a federal wanted list, without giving a reason, said Yarema Dukh from Ukrainian President Petro Poroshenko's press service.

"Vira to stay in the car, which is Ukrainian diplomatic car, until this issue to be clarified," Dukh said.

Poroshenko has alerted Ukraine's international partners about the incident, Poroshenko's office said.

It comes hours after the Russian news agency RIA reported that a procedure to extradite Nadezhda Savchenko had begun, quoting one of her lawyers.

A Russian court sentenced Savchenko to 22 years in jail on March 22 after finding her guilty of involvement in the killing of two Russian journalists during the separatist conflict in eastern Ukraine. She has denied any involvement.

"Vira Savchenko and our consul locked themselves in the diplomatic car. Russian authorities block the car ignoring its diplomatic status," Dmytro Kuleba, a spokesman for Ukraine's foreign ministry, wrote on Twitter.

"I hope the detention of Vira Savchenko doesn't mean Russia is taking a new hostage and we won't have to apply #FreeSavchenko to both sisters," Kuleba said.

Savchenko, 34, is regarded in her homeland as a national hero and symbol of resistance to Russia, which annexed Ukraine's Crimean peninsula in March 2014 after a Moscow-backed president was toppled during street protests in Kiev. Russia has also backed separatist rebels in eastern Ukraine.

Posters demanding her release are visible in many parts of Ukraine, including at Kiev's international airport and in parliament.

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But many in Russia see Savchenko as a Ukrainian nationalist with the blood of civilians on her hands. She was captured by pro-Russian forces in eastern Ukraine in June 2014 at the height of fighting there between Ukrainian forces and the separatists.

Poroshenko has said he and Russian President Vladimir Putin had agreed a framework for a deal to secure the release of Savchenko from prison.

The sentencing of two captured Russian servicemen in Ukraine earlier this month has fueled speculation that they might be swapped for Savchenko.

Vitaly Moskalenko, Ukraine's Consul General in the Russian port city of Rostov-on-Don, was expected to reach the border at around three in the morning local time (0000 GMT), Dukh said.

(Reporting by Matthias Williams; Editing by Andrew Hay, Toni Reinhold)