UN Secretary-General Ban Ki- moon on Wednesday highlighted the importance of cooperation in addressing the challenges facing the world, as leaders from major economies are to attend a G20 summit in Russia.

After arrived in the Russian city of St. Petersburg on Wednesday for the summit, Ban said the international community is facing great trials and tests, associate UN spokesman Farhan Haq told reporters at a daily briefing.

"Around the world, human rights are at risk. Democracies are threatened. Legitimate voices and movements of dissent are being stifled. People everywhere are worried about the future and wonder whether institutions and decision-makers will hear their pleas and act on them," Ban said in his remarks at St. Petersburg State University.

The UN chief said he firmly believed the international community had a duty to address the immediate crisis in the world, notably the civil war in Syria which has killed more than 100,000 people, displaced millions, and generated instability across the region, the spokesman said.

Ban is sure that G20 leaders would be heavily engaged in tackling the crisis, Haq said.

Ban also call on the international community to look to a wider time horizon and act now to take on the longer-term challenges, including strengthening global economic recovery and working to ensure sustainable development.

The UN chief has brought three inter-linked messages for G20 leaders: the need to accelerate work on the anti-poverty targets known as the Millennium Development Goals, to craft the post-2015 development agenda which is to build on the progress achieved by the MDGs and to tackle climate change, Haq said.