An al Qaeda-linked group has demanded the release of militants from Yemeni prisons, threatening to harm 73 soldiers it says it captured during a major assault in the south of the country earlier this week.

The threat from the Ansar al-Sharia group (Partisans of Islamic Law) was delivered via a text message sent to Reuters on Wednesday, three days after twin suicide bombings and clashes outside the southern city of Zinjibar killed 110 soldiers.

The mujahideen demand the release of their prisoners from the national and political security jails in exchange for saving the lives of 73 soldiers they hold captive in Abyan, the text message read.

In the event of failure to comply with their (the militants') demands, the lives of the soldiers will be in danger, it added.

The militants also urged relatives of the captured soldiers to lobby the U.S. ambassador to Yemen and newly-elected President Abd-Rabbu Mansour Hadi on the soldiers' behalf.

It was not immediately possible to verify the authenticity of the text message but militants have often used this method to communicate with local media in the past.

The army said on Monday some of its soldiers had gone missing but did not say how many. Hadi, who was elected last month, has said his forces will hunt the militants down.

Separately, the Interior Ministry said it had foiled a terrorist plot involving 300 members of al Qaeda who were planning a major attack on government facilities in the city of Mukalla on the Arabian Sea in Eastern Yemen.

They were planning to attack vital installations, military camps and security outposts and important government facilities with the aim of expanding the emirate of Azzan towards the city of al-Mukalla, the ministry said in a statement on its website, referring to an area in Shabwa province controlled by al Qaeda.

It said the militants were led by three prominent al Qaeda leaders it identified as Qasim al-Raymi and Egyptian-born Ibrahim al-Banna and Shaker Hamel, adding that security was being tightened in the province.

MILITANT LOSSES

Sunday's assault began with suicide bombers ramming vehicles laden with explosives at outposts outside the Abyan provincial capital of Zinjibar, followed by raids. Up to 20 militants were killed.

In a separate text message on Wednesday, the militants said they had allowed a small Red Cross medical team into Jaar, a town controlled under their control, to treat wounded government soldiers.

The Red Cross confirmed that a medical team had treated 12 soldiers injured in the fighting at a makeshift hospital in a Jaar school.

We managed yesterday mid-morning to obtain authorisation by (the) armed group Ansar al-Sharia to come and do some medical work in Jaar in Abyan province, Eric Marclay told Reuters, adding that the 12 soldiers were now in a stable condition.

We are performing life-saving surgery on the spot, he added in a statement.

Ansar al-Sharia is inspired by al Qaeda but the precise nature of its ties to the global network are unclear, although the Yemeni government says they are one and the same. Some analysts say it may be local militant groups at work however.

A year of political upheaval has severely weakened central government control over swathes of Yemen in favour of Islamist militants, who have expanded their foothold in the south, near oil shipping routes through the Red Sea.

Militants seized Jaar - the second largest town in Abyan - in March last year as protests against former President Ali Abdullah Saleh paralysed the country.

(Reporting by Mohammed Mukhashaf; Writing by Isabel Coles; Editing by Andrew Osborn)