Dec 22 (Reuters) - The United States is assembling a multinational naval coalition to help safeguard commercial traffic from attacks by Yemen's Houthi movement.

On Thursday, the Pentagon said more than 20 countries had now agreed to participate in the group, known as Operation Prosperity Guardian.

Some countries have not confirmed their participation, however, while others have said their efforts to help protect Red Sea commercial traffic will be as part of existing naval agreements rather than the new U.S.-led operation.

The lack of details and clarity over what countries are doing has added to confusion for shipping companies, some of which have been re-routing vessels away from the area after the attacks, which the Houthis say are a response to Israel's assault on the Gaza Strip.

WHAT HAS THE UNITED STATES ANNOUNCED?

U.S. Secretary of Defense Lloyd Austin announced on Tuesday plans to set up a multinational coalition to safeguard Red Sea shipping called Operation Prosperity Guardian.

During a trip to the Middle East, he said the operations would be joined by Britain, Bahrain, Canada, France, Italy, the Netherlands, Norway, Seychelles and Spain.

On Thursday, Austin said Greece and Australia had also joined the grouping, taking it to a total of 20, but added that at least eight countries taking part have declined to be publicly named.

WHAT ARE OTHER NATIONS SAYING?

- FRANCE

France's Defence Ministry said it supported efforts to secure freedom of navigation in the Red Sea and surrounding area and said it already operated in the region. But it said its ships would stay under French command and did not say if it would deploy more naval forces.

France has a naval base in the United Arab Emirates and 1,500 troops in Djibouti. Its frigate Languedoc is now in the Red Sea.

- ITALY

Italy's Defence Ministry said it would send naval frigate Virginio Fasan to the Red Sea to protect its national interests in response to specific requests made by Italian shipowners.

It said this was part of its existing operations and was not part of Operation Prosperity Guardian.

- SPAIN

Spain's Defence Ministry said it would only participate in NATO-led missions or EU-coordinated operations. "We will not participate unilaterally in the Red Sea operation," it said.

- BRITAIN

Britain said destroyer HMS Diamond would join Operation Prosperity Guardian. Britain's defence ministry said the coalition would operate as part of the U.S.-led CMF.

- GREECE

Greece said on Thursday it would send a naval frigate to the Red Sea and that it would participate in Operation Prosperity Guardian.

- OTHER COUNTRIES

The Netherlands said it would send two staff officers and Norway said it would send 10 naval officers to Bahrain, the headquarters of CMF. Denmark said on Wednesday that it would take part in the operation, sending one officer.

WHAT EXISTING NAVAL COALITIONS OPERATE IN THE AREA?

Several navies are already part of international operations to protect shipping lanes in the region, including protecting vessels from pirates who for several years disrupted shipping off the coast of Somalia.

The missions include:

- Operation Atalanta, set up by European Union Naval Force Somalia (EUNAVFOR), operates off the Horn of Africa and in the Western Indian Ocean to support U.N. resolutions to protect the seas from piracy. Its headquarters is in Spain.

- Operation Agenor is a European-led operation which aims to guarantee freedom of navigation in the Strait of Hormuz, a major shipping lane for oil exports from Gulf states.

- Combined Maritime Forces (CMF) is a multinational maritime partnership led by the U.S. from Bahrain, the based for the U.S. Navy Fifth Fleet. CMF has 39 members, including NATO and European states, regional countries and other nations. One of its missions is the Combined Task Force 153 , which operates in the Red Sea.

(Reporting by Reuters European bureaus; Writing by Dmitry Zhdannikov and Rachel Armstrong; Editing by Edmund Blair and Catherine Evans)