The United Nations (UN) peacekeeping force in Mali must remain in the country and at its current strength, UN Secretary-General Antonio Guterres has said in a new report, after Washington questioned its validity.

The presence of the mission remained crucial and its mandate remained relevant, given the complexity of the challenges, Guterres said in the quarterly report recently submitted to the Security Council.

He suggested that the mandate of the United Nations Multidimensional Integrated Stabilization Mission in Mali (MINUSMA) be extended for another year, until June 30, 2021, at the current overall strength.

The mandate for the force of more than 13,000 military and police personnel expires on June 30. The UN will vote on a resolution to extend MINUSMA on June 29.

For more than a year, the U.S., the largest financial contributor to the UN, has regularly questioned the validity of the mission, deeming it ill-suited to the ongoing violence in the West African nation.

The semi-arid country is struggling to contain an Islamist insurgency, that erupted in 2012 and h has claimed thousands of military and civilian lives since.

Despite the presence of thousands of French and UN troops, the conflict has engulfed the center of the country and spread to neighboring Burkina Faso and Niger.

"The human rights and humanitarian situations remain of deep concern," Guterres said.

He called for the implementation of a 2015 peace agreement to be accelerated.

And he warned that allegations of human rights violations against defense and security forces must be investigated.

(With input from AFP)