Gambia has been declared an Islamic Republic by president Yahya Jammeh, who insisted that the West African country will still "respect the rights of all citizens and non-citizens", regardless of their faith.

"In line with the country's religious identity and values, I proclaim Gambia as an Islamic state," Jammeh said on Friday.

"As Muslims are the majority in the country, Gambia cannot afford to continue the colonial legacy," he added.

Previously a secular country, Gambia joins the ranks of other Islamic Republics such as Afghanistan and Iran.

Its population of 1.8 million people is 95 per cent Muslim.

Jammeh is an animated orator who has earned the reputation for making surprise declarations over the course of his 21-year presidency. He pulled Gambia out of the Commonwealth in 2013, calling it neo-colonial, and in 2007 claimed to have found a herbal cure for AIDS.

Despite strong commercial ties with Britain and other European countries, relations with the West have deteriorated in recent years.

The European Union temporarily withheld aid money to the country last year over Gambia's poor human rights record. Gambia, whose main industries are agriculture and tourism, ranks 165 out of 187 countries on the UN development index.

"Starved of development funds because of his deplorable human rights record and economic mismanagement, Jammeh is looking toward the Arab world as substitute for and source of development aid," said blogger Sidi Sanneh, a former foreign minister who has become a US-based dissident.

Additional reporting by Reuters