The Saudi-led military coalition launched airstrikes on Yemen’s Houthi-held capital on Monday, the coalition and Yemeni security officials said, just two days after a foiled Houthi drone and missile attack targeting the Saudi capital.

The Saudi-led strikes appeared to signal an escalation in the coalition’s bombing campaign in Yemen.

The coalition has been fighting the Iran-backed Houthis there since March 2015.

Coalition officials said in a brief statement that they had started an operation to “destroy legitimate military targets” held by the Houthis. It did not elaborate further.

The officials said more than 12 airstrikes hit Houthi-held Sanaa on Monday, including six strikes on a military academy north of the capital and four on a military airbase that is within the Sanaa International Airport. Further airstrikes were reported in the northwestern Amran province.

There were no immediate reports of injuries, but residents said ambulances rushed to areas where the airstrikes took place.

Saudi Arabia’s Air Defense Forces on Saturday intercepted and destroyed a ballistic missile over the capital of Riyadh around midnight, according to state media. Another missile was also intercepted and destroyed over the southern Saudi city of Jizan, which borders Yemen.

The Houthis frequently launch missiles across Yemen’s border into Saudi Arabia, but the missiles rarely reach the capital. Riyadh is around 620 miles (1,000 kilometers) north of the border with Yemen.

Elsewhere in Yemen, military officials and tribal leaders said fierce clashes between Yemeni government forces and the Houthis in the mountainous northern province of Jawf and neighboring Marib killed more than 45 fighters from both sides in the past two days.

The officials said the Houthis attempted to seize control of the Sirwah district in Marib, but government forces, aided by airstrikes from the Saudi-led coalition, repelled the Houthi attack.

All officials spoke on condition of anonymity because they were not authorized to brief the media, and the residents and tribal leaders for fear of reprisal.

The U.N. Special Envoy for Yemen, Martin Griffiths, on Sunday voiced concerns about the escalation of fighting in Marib and Houthi attacks on Saudi Arabia.

“I am gravely dismayed and disappointed by these actions at a time when the Yemeni public’s demands for peace are unanimous and louder than ever before,” he said.

He called for the focus to be on “averting and mitigating the potentially disastrous consequences” of the coronavirus pandemic.

The recent escalation of fighting in the Arab World’s poorest country displaced more than 40,000 people since January, adding to the roughly 3.6 million who have fled their homes since the war began five years ago.

A number of those fleeing in recent weeks, including women and children, escaped on foot, walking for days without food or water across open desert, according to a recent statement by the United Nations Refugee Agency.

Yemen’s civil war erupted in 2014 when the Houthis seized Sanaa, along with much of the country’s north, ousting President Abed Rabbo Mansour Hadi. Months later the Saudi-led Arab coalition intervened to try and restore Hadi’s internationally recognized government.

The war has killed more than 100,000 people, many by Saudi-led airstrikes. It has led to the world’s worst humanitarian crisis, leaving millions suffering from food and medical shortages.

-AP