A Southern Popular Resistance fighter hides behind soldiers as they aim their weapons at Houthi positions around the airport of Yemen's southern city of Aden May 3, 2015. Reuters/Stringer

Yemen has asked the United Nations "to quickly intervene by land forces to save" the country from Shiite Houthi rebels, according to a letter from Yemen's U.N. Ambassador Khaled Alyeman. A Saudi Arabia-led coalition has been conducting airstrikes against Iran-allied Houthi rebels since late March.

The letter sent to the U.N. Security Council urged immediate action specifically in the cities of Aden and Taiz after Yemen's Houthi militia advanced into Aden's Tawahi district on Wednesday despite Saudi-led airstrikes killing 120 people. More than 1,200 people have been killed and 300,000 have fled their homes in the past several weeks, according to the U.N.

The letter reportedly stated that the rebels were "targeting anything that moves" in Aden. The letter also urged human rights groups to document "barbaric violations against a defenseless population,” accusing Houthis of killing several civilians and blocking medical teams in war-torn regions, according to Reuters.

Exiled Abed Rabbo Mansour Hadi, who is Yemen's internationally recognized president, appealed to the world to intervene to stop the Houthi assault on Aden. He called the rebels "the killers of men and children," adding that locals had appealed to Saudi Arabia for help "in the name of the brotherhood of blood and religion.”

Earlier this month, U.N. Secretary General Ban Ki-moon had expressed concern about the situation in Yemen calling for an immediate ceasefire in the region. Ban said that he was gravely concerned with the "continued ground fighting and aerial bombardment in Yemen and its impact on innocent civilians.

"There are credible reports of families in Aden trapped by the bombardment and snipers targeting civilians in the street," he had said, in a statement.