A jailed activist on a hunger strike since early February is in "good health" and receiving medical care, Bahraini officials have said, responding to claims from opposition groups that his condition was sharply deteriorating.

The statement by Bahrain's Interior Ministry came after social media postings raised alarms about the fate of Abdulhadi al-Khawaja, who has been on hunger strike since February 8.

Khawaja's wife, Khadija al-Mousawi, told the Reuters news agency her husband had failed to call on Tuesday from the military hospital where he is being monitored during his hunger strike and she was unable to obtain any information on his health on Wednesday.

"Something is very wrong," Mousawi said. "He was talking about accepting death as the path of freedom. He sounded very weak and tired," she added, referring to her last conversation with Khawaja on Monday.

Khawaja, 51, and seven other Shia activists were sentenced to life in prison last year.

The convictions were part of Bahrain's crackdown during the 14-month-old uprising by the country's Shia majority, which seeks to reduce the wide-ranging powers of the ruling Sunni dynasty.

The Interior Ministry statement described Khawaja as "in good health, despite rumors" and added that "he is in hospital, receiving full medical care".

Situation 'serious'

Ban Ki-moon, secretary-general of the United Nations, expressed concern over "continuing clashes" in Bahrain and called on authorities "to resolve Mr al-Khawaja's case based on due process and humanitarian considerations without any further delay", his office said.

On Monday, an appeals court delayed a decision until at least April 30 on efforts by Khawaja and others to challenge their convictions, which were issued by a military-led court.

Earlier this month, Bahrain rejected Denmark's request to take custody of Khawaja, who is also a Danish citizen.

On Monday, Danish Foreign Minister Villy Soevndal called the situation "very, very serious".

Khawaja has become a powerful rallying point for near daily protests in the strategic kingdom, which is home to the US Navy's 5th Fleet.

Western allies such as Britain and the US have offered only muted criticism of Bahrain for fear of alienating a trusted friend as well as its Saudi neighbour, which fears unrest could spread amongst Shias in the oil region of its Eastern Province.

In the latest violence, Bahraini authorities said an explosion wounded four police officers during clashes late Tuesday in one of the anti-government strongholds.

At least 50 people have been killed in Bahrain's protest-led violence since February 2011.