Ukrainian military pilot Nadiya Savchenko, convicted over the killing of two Russian journalists, began a hunger strike without water on Wednesday to demand her return to Ukraine

Ukrainian military pilot Nadiya Savchenko, convicted over the killing of two Russian journalists, began a hunger strike without water on Wednesday to demand her return to Ukraine, her lawyer said.

Lawyer Mark Feigin wrote on Twitter that Savchenko had started a "dry" hunger strike -- refusing both food and water -- on Wednesday morning, after posting a letter from her demanding "immediate return" to her homeland.

"She is not drinking," Feigin confirmed to AFP on Wednesday.

Experts say a human can survive several weeks without food but only a few days without water.

A Russian court last month found the 34-year-old pilot guilty of involvement in the fatal shelling of two Russian state television reporters in east Ukraine and sentenced her to 22 years in prison.

Savchenko has maintained her innocence but refused to appeal the verdict, saying she did not recognize the authority of the Russian legal system.

She has held several hunger strikes throughout her detention and high-profile trial.

Savchenko -- who has become a national hero in Ukraine and has been elected to parliament in absentia -- is currently incarcerated in a detention centre in southern Russia.

Her sentence formally entered into force on Tuesday and Russian law requires she leave for a prison camp within 10 days unless she is swapped or extradited to Ukraine.

Ukraine's pro-Western President Petro Poroshenko spoke to Savchenko by telephone on Tuesday and informed her on "the latest steps taken for her release," a statement on his website said.

Poroshenko told Savchenko that "justice will certainly prevail," the statement said.