At least 16,000 foreign nationals remain stranded in strife-torn Yemen, the International Organization for Migration (IOM) said Sunday,as it announced that it had evacuated 143 people from the capital Sanaa to Khartoum, Sudan.

The IOM said it had flown a first planeload of foreign nationals out of Yemen, where the bombing of Houthi rebels by a Saudi-led coalition has now entered a third week.

“The operation was a success and paves the way for continuing the evacuations of more than 16,000 third-country nationals who are stranded in Yemen,” IOM said in a statement.

Those evacuated included nationals from the United States, Sudan, Ethiopia, Nigeria, South Korea, Syria, Iraq, Indonesia, and several European countries. The organization said 38 countries had asked it to help evacuate their citizens stranded in the conflict in Yemen.

"The [humanitarian] situation is complex [and there] has been air strikes all day long," an Oxfam worker identified only as Nuha told Al Jazeera from Sanaa on Sunday.

Sunday's evacuation came as United Nations chief Ban Ki-moon urged the warring parties to return to the negotiating table. Speaking on the sidelines of a U.N. crime conference in Qatar's capital Doha, Ban said the fighting in Yemen should not be allowed to grow into a protracted regional conflict.

“There should be a cessation of military moves as soon as possible,” Ban said.

“Let the peace process be resumed, the United Nations stands ready.”

Saudi-led air strikes continued in Yemen on Sunday, entering their 18th day of attacks on the Houthi rebels backed by security forces loyal to former president Ali Abdullah Saleh. In February, they put President Abd Rabbu Mansour Hadi under house arrest, demanding that he carry out political reforms. Hadi, who fled the presidential palace to his power base in the southern city of Aden, has since sought sanctuary in Saudi Arabia, whose foreign minister said on Sunday that the air strikes sought to defend “legitimate authority.”

More than 600 people had been killed so far in the conflict and 2,000 injured, Ban said. NGOs have decried the onset of a humanitarian crisis in a country where 60 percent of its citizens lived below the poverty line before the start of the crisis.

“I am also deeply concerned about the military escalation where civilian casualties are mounting and public infrastructure is being destroyed,” he said.

Al Jazeera and wire services