German President Joachim Gauck has cancelled his summer trip to Ukraine amidst concerns over the health of imprisoned opposition leader Yulia Tymoshenko.

Tymoshenko, 51, has been on a hunger strike for nearly a week, after alleging guards punched her in the stomach and abused her body as they transported her to a local hospital to treat a spinal condition.

Tymoshenko had refused the treatment, fearing doctors may poison her, according to Reuters.

Gauck decided to cancel his visit to the Ukrainian resort of Yalta after discussing the issue with Chancellor Angela Merkel, German daily Süddeutsche Zeitung reported.

Gauck was set to attend a summit of central European leaders there next month.

While the German presidential role is largely ceremonial, the cancellation has symbolic significance, coming only months before the European football championships in Ukraine and Poland this summer.

Tymoshenko is serving a seven-year jail term for abusing her power while serving as the country's prime minister in 2009.

She is also facing a separate trial for embezzlement and tax evasion, and if found guilty could see her prison sentence extended by a further 12 years.

The opposition leader denies the charges, saying they are part of a smear campaign to silence the Ukrainian opposition movement.

German doctors treating Tymoshenko in her prison in the eastern city of Kharkiv have diagnosed her with a slipped disc.

Ukrainian authorities have denied the doctor's requests to have her flown to Germany for treatment.

Elsewhere on Thursday, European Union foreign policy chief Catherine Ashton said that she was deeply preoccupied with the plight of Tymoshenko.

Bowing to international criticism, Ukraine's President Viktor Yanukovych ordered an investigation into the allegations of brutality on Thursday.