The United States on Sunday condemned an aerial attack against Saudi Arabia’s capital, saying it appeared to be “an attempt to target civilians.” 

Saudi state media reported the kingdom intercepted an apparent missile or drone over Riyadh on Saturday. 

Houthi rebels in neighboring Yemen, who have launched numerous cross-border attacks since Saudi Arabia began leading a coalition in support of Yemen’s internationally recognized government in 2015, denied any involvement. 

The U.S. State Department said attacks like the one Saturday go against international law and “undermine all efforts to promote peace and stability.” 

“As we work to de-escalate tensions in the region through principled diplomacy, including by bringing an end to the war in Yemen, we will also help our partner Saudi Arabia defend against attacks on its territory and hold those who attempt to undermine stability to account,” it said in a statement. 

The Saudi-led coalition entered Yemen’s conflict months after the Houthis seized the capital, Sana’a.  It has faced international criticism for airstrikes that have killed hundreds of civilians and struck non-military targets. 

The United Nations said Sunday that it along with the International Committee of the Red Cross are facilitating talks on a new prisoner swap between Yemen’s warring sides. 

The latest effort at some progress in resolving the conflict comes two weeks after the outgoing U.S. administration designated the Houthis as a terrorist organization. 

Aid groups had warned such a move could seriously harm efforts to get badly needed food and other supplies to civilians in the world’s worst humanitarian crisis. 

A group of 22 groups, including the Norwegian Refugee Council, Oxfam and Save the Children, warned Sunday of a “potentially catastrophic humanitarian impact” and said the designation should be “revoked immediately.” 

Antony Blinken, President Joe Biden’s secretary of state nominee, told lawmakers at a hearing last week that the State Department has launched a review of the matter.