ISTANBUL (Reuters) - U.N. second-in-command Jan Eliasson said on Sunday it was necessary to stand up for international humanitarian law which was being disregarded, citing attacks on hospitals and “practically medieval” sieges on civilians in Syria and Yemen.

United Nations Deputy Secretary-General Jan Eliasson speaks during a news conference before the U.N. World Humanitarian Summit in Istanbul, Turkey, May 22, 2016. REUTERS/Osman Orsal

“We have to stand up for international humanitarian law. We have seen a decay, a lack of respect for international law which is causing enormous damage in the world,” Eliasson said at the opening news conference of the first world humanitarian summit. He added that there were many signs the Geneva convention on human rights was being neglected.

Government and business leaders, aid groups and donors gather in Istanbul for the summit this week to try to develop a more coherent response to what U.N. chief Ban Ki-moon has called the worst global humanitarian situation since World War Two.

Eliasson said attacks on civilian targets, hospitals and schools in Syria, Yemen and Afghanistan were examples of disregard for the Geneva convention on human rights. He said sieges in Syria in Yemen where people were being isolated in “absolute violation of humanitarian law” which was “practically mediaeval.”

This month the U.N. accused the Syrian government of withholding aid to hundreds of thousands of people and risking a new siege, saying it had appealed to the government to change its mind.

In Yemen, the UN has previously accused Iranian-allied Houthi militia of obstructing the delivery of humanitarian supplies to civilians in the southwest, saying residents had been living under “virtual siege”.

According to UNICEF, the U.N.’s children’s agency, an average of four schools or hospitals are attacked or occupied by armed forces and groups every day.

Last year, 75 hospitals managed or supported by respected medical NGO Medecins Sans Frontieres (MSF) were bombed, the NGO said, adding civilians were being wounded and killed by indiscriminate warfare in Syria, Yemen, South Sudan, Afghanistan and elsewhere.

However MSF pulled out of the two-day summit, which aims to mobilize funds and get world leaders to agree on issues ranging from how to manage displaced civilians to renewing commitments to international humanitarian law, saying the “non-specific good intentions to ‘uphold norms’ and ‘end needs’” of the summit would mean humanitarian violations would be ignored.

Eliasson said he admired MSF and was surprised that it decided to stay away, and said he hopes the leadership of the aid agency would see that this conference serves its purpose:

“We will very much make their issues the issues of this conference.”