RABAT, March 21 (Reuters) - Moroccan authorities have arrested Kassim Tajideen, deemed by Washington a top financier of the Lebanese Hezbollah movement, and plan to extradite him to the United States, a Moroccan security source close to the case said on Tuesday.

Tajideen was arrested on March 12 at Casablanca airport on an arrest warrant issued two days earlier by Interpol's Washington office for alleged fraud, money laundering and financing of terrorist activities, the source told Reuters.

He was in transit from Guinea's capital, Conakry, on his way to Beirut, said the source, who was not authorized to speak to the media.

Morocco's Interior Ministry declined to comment and the Ministry of Justice did not immediately respond to request for comment.

The U.S. Department of Treasury designated Tajideen, a Lebanese and Sierra Leonean national, a Specially Designated Global Terrorist in May 2009, calling him "an important financial contributor to Hezbollah".

In 2010, the Treasury Department hit Tajideen's brothers, Husayn and Ali, with sanctions for allegedly providing support to the Iranian-backed group. It said that Ali was once a Hezbollah commander in Lebanon.

U.S. sanctions have targeted the brothers' network of businesses in real estate, construction, diamonds and the food trade in Gambia, Sierra Leone, Democratic Republic of Congo, Angola and the British Virgin Islands.

Reuters was unable to reach members of the Tajideen family for comment. Hezbollah has previously denied U.S. accusations that it is linked to money-laundering.

The U.S. embassy in Rabat had no immediate comment.

The U.S. sanctions in 2010 barred U.S. citizens from doing business with the brothers but some of their companies have continued to operate.

The Interpol warrant against Kassim Tajideen accused him of making illegal money transfers worth $27 million through a business network that runs through Angola, the United Arab Emirates and Lebanon, the source said.

U.S. officials have long been concerned that central and west African countries could become safe havens for financiers of Hezbollah looking to take advantage of weak financial regulation.

Hezbollah has played an important role in Syria's civil war in recent years, lending crucial firepower to President Bashar al-Assad's forces against rebel groups.

(Reporting by Samia Errazzouki; Writing by Aaron Ross) ((Aaron.Ross@thomsonreuters.com; +243 81 51 00 107;))