Saudi Arabia said its air defenses had taken down a rocket aimed at residential areas, while the rebels said they had been targeting a royal palace.
BEIRUT, Lebanon — Rebels in Yemen fired a ballistic missile on Tuesday at Riyadh, the capital of Saudi Arabia, for a second time in two months, though Saudi officials said that it had been intercepted and that there were no casualties.
Around midday, a large boom startled many in Riyadh, including customers at a cafe in the center of the city, where several ran outside to see a puff of gray smoke in the sky and black smoke rising from the ground nearby, presumably from the launch site of the defense systems.
The Saudis are at the forefront of a coalition that has been waging a bombing campaign in Yemen for two and a half years against the rebels, known as the Houthis, that has contributed to a humanitarian crisis that United Nations officials consider among the worst in the world.
Saudi Arabia has accused Iran of backing the Houthis with weapons and expertise, concerns shared by the United States government, and the Saudis said on Tuesday that they had been targeted by an “Iranian-Houthi missile.”
A spokesman for the Saudi-led military coalition said the missile had been aimed at residential areas and had been intercepted “without any casualties,” according to the state-run Saudi Press Agency. The spokesman, Turki al-Maliki, called the possession of such weapons by militant groups like the Houthis “a threat to regional and international security.”
The Houthis acknowledged firing the missile but said the target had been the a palace of King Salman, the Saudi monarch, according to their television station, Al Masirah.
The attack came hours before King Salman was to lead a ceremony to announce the kingdom’s 2018 budget, and the timing suggested that the Houthis were attempting to spread fear in the capital and draw attention away from the Saudi leadership’s plans for governance and development.
The war in Yemen began in 2014 when the Houthis allied with parts of the Yemeni armed forces and seized much of the country’s northwest, including the capital, Sana, later forcing the government into exile. In 2015, Saudi Arabia and other Arab states began a bombing campaign aimed at pushing the Houthis back and restoring the internationally recognized Yemeni government.
Throughout the war, the Houthis and their allies have targeted communities along the Yemen-Saudi border with missiles and other projectiles, but they appear to be expanding their range.
The Houthis fired a missile at Riyadh’s international airport on Nov. 4. Saudi officials said their missile defenses had brought it down, but an investigation by The New York Times found that the missile had hit near its target and that it was unclear whether the body of the projectile had been struck.
Last week, Nikki R. Haley, the American ambassador to the United Nations, stood in front of what United States officials said were Iranian-made missiles, including the one that was fired at Riyadh’s international airport in November. But Defense Department officials said they doubted that the remnants on display validated Ms. Haley’s claims.
The strike on Tuesday came amid an increase in violence in Yemen, after the death on Dec. 4 of Ali Abdullah Saleh, a former president who had allied with the Houthis and brought allied military units with him.
Shortly after Mr. Saleh had suggested the possibility of turning a “new page” with Saudi Arabia, the Houthis killed him and some of his aides in an attack on their convoy.
United Nations officials said on Tuesday that airstrikes by the Saudi-led coalition had killed scores of civilians since Mr. Saleh’s death, and they voiced alarm at the humanitarian effect in Yemen of Saudi restrictions on imports to the country.
With Yemen grappling with health emergencies and the risk of famine, the United Nations said it had verified the deaths of at least 136 civilians in airstrikes on Sana and on several other areas from Dec. 6 to Dec. 16, according to Rupert Colville, a spokesman for the United Nations human rights office in Geneva.
The heaviest casualties occurred when coalition aircraft bombed a military police compound in Sana, hitting a prison building and killing at least 45 people.
The victims of that strike were all reportedly captive members of forces loyal to the Saudi-backed president in exile, Abdu Rabbu Mansour Hadi.
The United Nations also expressed concern about Houthi attacks on members of Mr. Saleh’s political party, including reports of summary killings and detentions of people affiliated with it.
Verifying those incidents was difficult, the United Nations said, because witnesses were afraid of Houthi retaliation if they spoke out.
Still, airstrikes by the Saudi-led coalition in that period of December accounted for the overwhelming majority of the civilian casualties, as they have throughout the war, Mr. Colville said.
In a one-week period up to Dec. 14, coalition airstrikes killed 93 civilians, while military action by Houthi forces had killed three people, he said.
International aid agencies are increasingly expressing worries about obstacles to the delivery of fuel and aid caused by the tight blockade Saudi Arabia imposed on Yemen after the first missile attack.
The blockade was eased in response to international pressure, and some shipments of food and medicine have been unloaded, but United Nations officials say the obstruction of commercial supplies is driving the country deeper into what is already one of the world’s most acute humanitarian disasters.
Shortages of fuel in particular are deepening the health crisis in a country suffering through the worst cholera outbreak in modern history, affecting close to a million people and resulting in 2,225 deaths. Aid agencies say nearly 400,000 children suffering severe acute malnutrition are fighting for their lives.
Water pumping stations serving over three million people in 14 cities are now running out of fuel and safe drinking water, which are crucial for tackling cholera and malnutrition but unaffordable for two-thirds of Yemen’s population, said Christophe Boulierac, a spokesman for the United Nations children’s agency.
