By Felix Onuah

ABUJA (Reuters) -Nigeria's President Bola Tinubu made sweeping changes to the defence forces on Monday, forcing out the security chiefs and the head of police less than a month after taking office.

Tinubu, who was sworn in on May 29, has made security one of his major priorities and promised reforms to the sector, including recruitment of more soldiers and police officers, while paying and equipping them better.

Nigeria's military is stretched fighting a long running Islamist insurgency in the northeast and banditry and kidnappings for ransom in the northwest as insecurity has spread to most parts of the country.

It is not unusual for a new Nigerian president to send security chiefs into early retirement upon taking office, as Tinubu did on Monday.

He picked Nuhu Ribadu, a former senior police officer and ex-head of the country's economic and financial crimes agency, as his National Security Adviser.

Major General Christopher Musa, who until last year was leading the army's fight against the insurgency, takes over as the new Chief of Defence Staff from Lucky Irabor.

Musa takes over leadership of the military in the aftermath of a Reuters report last year that the Nigerian Army ran a secret and illegal abortion programme targeting women and girls who had been kidnapped and raped by Islamist militants.

Musa denied the programme ever existed and said Reuters reporting was part of a foreign effort to undermine the country's fight against the insurgents.

Tinubu also named new commanders for the army, navy and air force as well as a new head of the Nigeria Customs Service with immediate effect.

(Reporting by Felix Onuah, additional reporting by Camillus Eboh; Writing by MacDonald Dzirutwe;Editing by Alison Williams and Marguerita Choy)