His intervention came after Iran and the United States once again traded fire following the downing of an American helicopter, further straining a ceasefire that took effect in April but has been marked by sporadic flare-ups of violence.

"We should not minimize the risks of a lesser fire becoming full fire, or in another word -- full war," Secretary-General Guterres said at a meeting of the UN Security Council devoted to the situation in the Middle East.

The UN rights chief echoed Guterres, saying he was "horrified by the fact that we see escalation upon escalation."

"We have, I mean, we're always very relieved when ceasefires are announced, but ceasefires need to be respected in full. International law needs to be respected in full," said the UN High Commissioner for Human Rights Volker Turk in Geneva.

The war, which began with US-Israeli strikes on Iran, threw the region into chaos and rattled global markets before the shaky truce began.

Iran said it attacked American bases in Jordan and Bahrain on Wednesday after US forces carried out strikes on the Islamic republic in retaliation for the downing of a helicopter.