German authorities have played down the likelihood that a terrorist attack caused a blast on a ferry with hundreds of passengers on board in the Baltic Sea in the middle of the night.

"The investigation is continuing, but the first indications are that it was most likely caused by a technical fault on a lorry," Stefan Jung, police spokesman in the northern German city of Kiel, told AFP.

In total, 249 people were evacuated after a blast on the upper deck of the Lisco Gloria ferry caused a fire shortly after midnight (2200 GMT), the spokesman said.

Twenty-eight people needed medical treatment, and 23 were still in hospital on Saturday, all suffering from smoke inhalation. None of those on board had serious injuries.

The passengers on the Lithuanian-flagged car ferry, which had left Kiel for the Lithuanian port of Klaipeda, included Germans, Russians, Danes, Lithuanians, Latvians and Argentinians. They were all taken back to Kiel.

An inquiry had been launched to determine the cause of the accident, but the coastguards stressed there was "absolutely no" indication of an attack.

The United States, Britain, Japan and Sweden recently issued alert warnings of a "possible terrorist attack" by Al-Qaeda and affiliated groups against their citizens travelling in Europe.

This followed reports in British and US media that said that an Al-Qaeda plot to carry out attacks in Britain, France and Germany, similar to the assault in Mumbai in 2008 that killed 166 people, had been uncovered.

A US drone attack on Monday in Pakistan killed five German nationals said to be undergoing training to fight with the Taliban or Al-Qaeda.