Boys sit on their house belongings as they are transported on a pick-up vehicle while returning home to al-Jadaan area, after the pro-government army retook the area from Houthi rebels, in Yemen's central province of Marib, December 21, 2015. [Photo/Agencies]

The special envoy also noted the worsening security situation in Yemen, saying that the residents of Taiz continue to suffer the consequences of heavy fighting and a severe lack of assistance. He said that the conflict and the security vacuum it caused have led to a dangerous expansion of extremist groups in the country.

The situation in Yemen has substantially deteriorated since the conflict broke out in early 2015, with a 9 percent increase in the internally displaced persons (IDPs), which have reached 2.5 million, according to a recent United Nations-backed report on the issue.

"The ongoing conflict, damage to civilian infrastructure, and strain on already depleted resources have exacerbated an already precarious humanitarian situation," said Johannes van der Klaauw, the representative of the Office of the UN High Commissioner for Refugees (UNHCR) in Yemen, highlighting the latest report of the Task Force on Population Movement (TFPM), which was led by UNHCR and the International Organization for Migration (IOM) and partners.

The main needs of IDPS, according to the report, are food, water, sanitation and hygiene, and shelters. Most IDPs have lost their livelihoods and have sought shelter with relatives and friends, in schools, public and abandoned buildings and makeshift shelters or in the open with little to no protection.

Last week, the UN World Health Organization and its partners appealed for 31 million U.S. dollars to ensure the continuity of medical services for nearly 15 million Yemenis following the collapse of the country's health system.

Endemic food shortages, along with reduced access to health facilities and sanitation, have further been compounded by the lack of fuel, electricity, gas and water.

Yemen has mired in political gridlock since 2011 when mass protests forced former President Ali Abdullash Saleh to step down.

The ongoing crisis in conflict-stricken Yemen is reflection of a regional unrest in the Middle East, especially after the forces of fleeing President Abd-Rabbo Mansour Hadi seized strategic southern city of Aden against Shiite Houthi fighters, reports said.

The Shiite Houthi group launched attacks on Aden city, which President Abd-Rabbu Mansour Hadi declared as temporary capital after he fled weeks of house arrest by the Houthis in Sanaa.

On March 26, a Saudi-led coalition started airstrikes on Houthi targets in Sanaa and other cities, saying the multinational action was to protect Hadi's legitimacy and force the Houthis to retreat from cities it seized since September 2014.