SANAA, Yemen (AP) -- Street battles and air raids are driving more and more Yemenis from their homes, the United Nations said Tuesday as the worsening conflict forced the impoverished Arabian Peninsula country's liquefied natural gas company to shut down production.

The U.N. Office for the Coordination of Humanitarian Affairs said estimates show that over 121,000 people had been displaced inside Yemen since a Saudi-led coalition airstrike campaign targeting Shiite rebels began on March 26, with nearly half that number concentrated in the country's northwest.

"Humanitarian partners are providing assistance, including water, sanitation and health services, but the response is constrained by continued high insecurity due to airstrikes and fighting on the ground," spokesman Jens Laerke said.

The figures were released as the Security Council adopted a resolution placing an arms embargo on the leaders of the Shiite rebels, known as Houthis, and their key allies, ordering them to end the violence and return to U.N. brokered negotiations.

A combined force of Shiite rebels and security forces loyal to Yemen's former president, Ali Abdullah Saleh, have taken over large swaths of the country, including the capital, Sanaa, and are bearing down on the southern port city of Aden, where troops loyal to the internationally recognized government are struggling to hold them off in street battles.

Earlier Tuesday, Iran's foreign minister called for a peace plan that includes humanitarian aid, dialogue and the formation of a broad-based Yemeni government — with no preconditions as to who would run Yemen imposed before negotiations between the country's different factions begins. Speaking in Madrid, Foreign Minister Javad Zarif did not address Saudi claims that Tehran has been giving the Houthis military support, which both Iran and the rebels deny.

On the ground in Yemen, fighting continued, with tribal forces loyal to the internationally recognized government advancing on a military base held by Saleh's supporters outside the central city of Marib. The fall of the base would clear a path to the rebel-held capital.

At a press briefing in the Saudi capital of Riyadh, coalition spokesman Brig. Gen. Ahmed Asiri confirmed that the base had been targeted. He also welcomed the U.N. Security Council resolution, saying that diplomatic as well as military pressure was needed "to achieve peace" in Yemen.

Asked about whether Egyptian ground troops would be taking part in Yemen operations, Asiri said that "so far, we cannot comment on any question about future operations," but added that the coalition is keeping all options on the table. Asiri spoke just hours after Saudi Defense Minister Mohammed bin Salman arrived in Egypt for meetings with President Abdel-Fattah el-Sissi on the conflict.

In Yemen's Omran province north of Sanaa, military officials said airstrikes hit one of the Houthi's main brigades. And near the southern city of Aden, local fighters and airstrikes destroyed a convoy of pro-rebel fighters, witnesses and security officials said. All spoke on condition of anonymity, either because they feared retribution or were not authorized to talk to reporters.

The Yemeni natural gas company, whose main shareholder is French oil giant Total SA, said the violence forced the shutdown of its plant in the oil-rich central Shabwa province, where the rebels last week seized the capital, Ataq, after enduring coalition airstrikes and fierce battles with rival tribes. Yemen is already suffering from electricity shortages, with Sanaa without power since Sunday, and the plant's closure will likely exacerbate the situation.

Shabwa is also a stronghold of Yemen's local al-Qaida branch, which residents say recently launched a bloody ambush on rebels in Ataq.

A video obtained by The Associated Press and consistent with its reporting in the area showed bodies of over a dozen rebels and allied troops killed in the attack. Residents found the bodies on the side of the road Tuesday. Pro-Houthi television channel al-Masirah confirmed the killings, putting the death toll at 15 and saying some were beheaded.

Later in the day, al-Qaida's Yemen branch said its top cleric, Saudi-national Ibrahim al-Rubaish, was killed in a drone attack two days earlier, but did not specify in its statement where the purported strike took place.

In Geneva, U.N. High Commissioner for Human Rights Zeid Raad al-Hussein urged all sides in Yemen to ensure that attacks resulting in civilian casualties are promptly investigated and that international human rights and humanitarian law are respected during the fighting.

In a statement, al-Hussein said that in addition to hundreds of fighters, at least 364 civilians have lost their lives since March 26, when the coalition air strike campaign began. At least 681 civilians, including at least 84 children and 25 women, have been injured. Dozens of public buildings, including hospitals, schools, airports and mosques, have been destroyed in airstrikes, through shelling and other attacks, Hussein added.

Also Tuesday, an aircraft loaded with 76 metric tons of medical supplies arrived in Sanaa, UNICEF said in a statement. The UNHCR also said that refugees continue to arrive in Djibouti and Somalia from Yemen, with a total of 1,260 people arriving by boat to both countries over the past two weeks.

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Rohan reported from Cairo. Associated Press writer Aya Batrawy in Riyadh, Saudi Arabia, contributed to this report.