The release of six kidnapped U.N. aid workers held by tribesmen in Yemen has been delayed because of a dispute over an exchange with a prisoner held by police, a tribal source said on Thursday.

The government had earlier said the aid workers - a German, a Palestinian, an Iraqi, a Colombian and two Yemenis who were seized on Tuesday - had been released, but the tribal source said the kidnappers had made a last minute demand for a fellow tribesman held by police be handed over to them first.

Efforts to release the hostages, who work for the U.N.'s Office for the Coordination of Humanitarian Affairs (OCHA), were continuing, said the source.

Kidnappings are common in Yemen where hostages are often used by disgruntled tribesmen to press their demands on authorities, and are usually freed unharmed.

One of the kidnappers had earlier said there were also delays due to one of the hostages falling ill. He said the hostage's health had improved after receiving medical treatment.

The United States and neighbouring Saudi Arabia are worried that political unrest could give al Qaeda's Yemen wing the opportunity to gain more control of the impoverished country.

In southern Yemen, five Islamist militants were killed in clashes with government soldiers, a security source said, in escalating violence ahead of a presidential election this month.

Protests have continued even after President Ali Abdullah Saleh transferred his powers and bowed to a year of mass protests demanding the end of his 33-year rule.

Activists demand that Saleh, in the United States for medical care, be tried for alleged killings of protesters, and the government be purged of his relatives.

Yemenis are due to go to the polls on February 21, to vote for a

new president. Many fear that a wave of violence in Sanaa could escalate to derail the vote.

The defence ministry said late on Wednesday that a leading member of the security forces escaped an assassination attempt in Sanaa.

(Reporting by Mohammed Mukhashef and Nour Merza Additional reporting by Mohammed Ghobari in Sanaa; Writing by Nour Merza)