ISTANBUL — Saudi Arabia, stung by growing international criticism over its role in Yemen's worsening humanitarian crisis, said on Monday that it would donate $1.5 billion in aid to relief organizations and create "safe-passage corridors" to ensure the delivery of goods across the war-stricken country.

The announcement of expanded relief measures comes nearly three years after a Saudi-led military coalition intervened in Yemen's civil war to try to defeat the Houthis, a rebel group that ousted Yemen's government. But neither side has been able to win significant territory in a grinding conflict that has produced misery for civilians, who face peril from airstrikes and rocket fire, a cholera outbreak and shortages of food and fuel.

The United Nations and human rights organizations have accused all parties in the war of imposing hardships on civilians. But the Saudi-led coalition, which has received U.S. military support during the conflict, has faced added criticism because it controls access by land, air and sea to Yemen and has imposed what aid organizations say is a de­ facto blockade that has starved the country, which is heavily reliant on imports of essential supplies.

Airstrikes by the Saudi-led coalition are responsible for killing the majority of the more than 5,000 civilians who have died in the war, according to a U.N. report released in August. Saudi officials have blamed the Houthis for the lack of humanitarian access in Yemen and denied that their airstrikes disproportionately target civilians.

But the critiques, including from stalwart allies such as the United States, had started to bite.

In December, President Trump told Saudi Arabia to "allow food, fuel, water and medicine to reach the Yemeni people who desperately need it" — a rare rebuke that served to corroborate accusations by aid agencies that the Saudis had effectively sealed off Yemen.

The measures announced Monday included an initiative to expand Yemeni ports to increase the supply of commercial imports and an "air bridge" to a Yemeni city under the control of the Saudi-led coalition that would accommodate up to six cargo flights a day and that would "be available for use by humanitarian organizations to deliver critical humanitarian needs," according to a Saudi statement.

The safe-passage corridors — including routes through Houthi-controlled territory — would be placed on no-strike lists that exempt them from targeting by coalition warplanes, the statement said.

"We are backing a professionally planned and detailed humanitarian mission with military power and precision to guarantee that the humanitarian aid reaches the people who need it," Col. Turki bin Saleh al-Malki, a coalition spokesman, said in the statement.

The Saudi promises of financial aid and assurances that aid corridors would be off-limits to airstrikes were "welcome steps," said Scott Paul, senior humanitarian policy adviser at Oxfam America. But the coalition needs to take additional measures, he said, including allowing commercial flights to land in Sanaa, the Houthi-controlled capital, and ensuring open access to Yemen's most important seaports, including the rebel-
controlled port in the city of Hodeida.

None of the measures, however, would be sufficient to stave off growing civilian hardship without a cease-fire, Paul said.

But in the past few months, the conflict has tended toward greater violence.

Since the Houthis broke with a key ally in December, coalition forces have redoubled their efforts to advance toward Sanaa, sensing an opportunity.

"We need a political settlement to get out of this," Paul said. "Everyone needs to put down their weapons."