A hunger strike by Ukrainian pilot Nadia Savchenko, who is being held in Russian custody, has entered its fourth week.

Savchenko's legal team says she has been ingesting only warm water for the past 21 days and that her health has begun to suffer.

Her lawyers have been barred from visiting Savchenko in detention through the end of state holidays on January 12.

Savchenko, who was captured by pro-Russian separatists in eastern Ukraine in June, was transferred to Russian pretrial detention in July.

Russian authorities have charged the 33-year-old pilot with complicity in the killing of two Russian journalists covering the Ukraine conflict.

Savchenko denies the charges and says her transfer to Russia was illegal.

Savchenko's supporters on January 5 are carrying out a one-day Twitter campaign, using the hashtag #FreeSavchenko, to mark the 22nd day of her hunger strike.