(Corrects to make clear that graft probe was conducted by GAVI)

Feb 25 (Reuters) - Niger arrested about 20 doctors suspected of embezzling funds from a charity set up by the Bill and Melinda Gates Foundation to promote vaccination in poor countries, judicial and police sources said on Monday.

Niger's government has made tackling corruption a priority since taking office in 2011, and last year President Mahamadou Issoufou fired two ministers suspected of illegally awarding state contracts.

The doctors' arrests came after an investigation by the GAVI Alliance - backed by the Bill and Melinda Gates Foundation, the World Health Organization, the World Bank, UNICEF, donor governments and others - which centred on some $1.5 million donated between 2007 and 2010.

"Some 20 doctors and public health managers were the object of arrest warrants and therefore jailed following an investigation into embezzlement of GAVI funds," said a judge in Niamey, who asked not to be named.

The arrests were made over the weekend.

GAVI had suspended its financing of programmes in Niger until the embezzled money was reimbursed. The charity says Niger's government has agreed to repay the embezzled money and is in the process of doing so.

The non-profit organisation, which aims to improve access to immunisation in the world's poorest countries, was launched in 2000 with a $750 million grant from the Gates foundation.

In December, GAVI suspended $6 million in funding to Sierra Leone after an audit showed misuse of $1.1 million of previously disbursed funds, including undocumented expenses, cash handouts and overcharged procurement costs between 2008 and 2011.

Landlocked Niger, one of the poorest countries the world, has about 850 public doctors for its 17 million people.

Earlier this month, GAVI - which funds bulk-buy vaccination programmes - said Niger would be one of eight countries in Africa and Asia taking part in a pilot project to immunise 180,000 girls against cervical cancer. (Reporting by Abdoulaye Massalatchi; Editing by Louise Ireland)