Yemen´s warring parties failed to operate the first commercial flight in six years from the rebel-held capital on Sunday, dealing a blow to an already fragile truce in the country´s grinding conflict.

The flight to Amman, Jordan, had been planned as part of the U.N.-brokered, 60-day truce agreement that the internationally recognized government and the Houthis struck earlier this month.

The truce, which went into effect on April 2, is the first nationwide cease-fire in Yemen in six years. It came amid concerted international and regional efforts to find a settlement to the conflict that devastated the Arab World´s poorest country and pushed it to the brink of famine.

Yemen´s brutal civil war erupted in 2014, when the Iranian-backed Houthis seized the capital, Sanaa, and forced the government into exile. The Saudi-led coalition entered the war in early 2015 to try restore the government to power.

The conflict has in recent years become a regional proxy war that has killed more than 150,000 people, including over 14.500 civilians. It has also created one of the worst humanitarian crises in the world.

As part of the truce, the two sides agreed to operate two commercial flights a week to and from Sanaa to Jordan and Egypt. Sanaa is blockaded by the Saudi-led coalition.

However, both sides have failed to operate the first flight, over three weeks since the truce took effect. They have traded blame for the failure.

The Houthis accused the Saudi-led coalition of failing to issue needed permits for the flight, without giving further details.

Moammar al-Iryani, information minister of the internationally recognized government, said the Houthis did not adhere to the agreement because they wanted dozens of passengers to board using passports issued by the Houthis.

He said the government allowed the travel of 104 passengers on the Sanaa-Amman flight but the Houthis insisted on adding 60 more passengers "with unreliable passports."

The internationally recognized government announced in March 2017 that it doesn't recognize documents issued by the Houthis.

A spokesman for the Houthis did not respond to a request for comment.

Hans Grundberg, the U.N. special envoy for Yemen, urged both sides to "work constructively" with the U.N. to address the challenges that delayed the flight.

"The Truce is meant to benefit civilians including through reducing violence, making fuel available, and improving their freedom of movement to, from and within their country," he said on Twitter.

The U.N. envoy's office said Grundberg began mediation efforts to address differences between the two sides on flight procedures when a disagreement arose on Thursday. It did not elaborate.

-Agencies