United Nations Secretary-General Antonio Guterres warned on Thursday that the UN Charter is under assault like never before as the 193-member world body marked the 80th anniversary of the signing of its founding document.

"We see an all too familiar pattern: Follow when the charter suits, ignore when it does not. The charter of the United Nations is not optional. It is not an a-la-carte menu. It is the bedrock of international relations," Guterres said.

Upholding the purposes and principles of the UN Charter is a never-ending mission, said Guterres, stressing the need more than ever to respect and re-commit to international law – in words and deeds.

"On this anniversary, I urge all member states to live up to the spirit and letter of the charter, to the responsibilities it demands, and to the future it summons us to build – for peace, for justice, for progress, for we the peoples," he said.

The UN Charter, the foundational treaty of the world body, was adopted on June 25, 1945, at the San Francisco Conference and was signed by delegates the following day.

(With input from Xinhua, Reuters)