Yemeni rebels have dramatically ramped up attacks on targets inside Saudi Arabia over the past month, complicating the Biden administration’s efforts to broker a peaceful resolution to Yemen’s years-long conflict and ease its humanitarian crisis.

Since mid-February, Yemen’s Houthi rebels, backed by Iran, have claimed responsibility for sending dozens of missiles and armed drones into Saudi Arabia on at least 13 days. Saudi Arabia, which leads a coalition of forces supporting the internationally recognized government, has been waging an air campaign blamed for thousands of civilian casualties in Yemen.

The Houthis’ targets have included the kingdom’s capital, Riyadh; Abha International Airport, roughly 125 miles north of the Yemeni-Saudi border; Jiddah Airport; and the King Khalid air base in southern Saudi Arabia. Last week, the rebels also claimed two attacks on Aramco oil facilities. In contrast, the Houthis did not claim responsibility for any strikes against Saudi Arabia in January.

Analysts said the Houthis may be escalating their attacks to gain leverage ahead of anticipated talks over ending the war. Biden’s special envoy, Timothy Lenderking, recently held discussions in the region regarding how a peace process could move forward.

Jamal Benomar, a former U.N. special envoy to Yemen, said that “everybody is anticipating there is going to be a negotiation, and that's why things are heating up.”

The White House has described ending the six-year war as an early priority for the new administration. Last month, U.S. officials announced they would end U.S. support for offensive operations by the Saudi-led coalition, which first intervened in 2015 after the Houthis overran Yemen's capital and swept the government from power.

The administration also announced plans to reverse an 11th-hour Trump administration decision to label the Houthis as a foreign terrorist group. Aid groups had warned this designation would interfere with their emergency efforts and the already-dire humanitarian situation. The Saudi-led coalition says the rebels have been emboldened by the reversal of the designation.

Senior Houthi official Mohammed Ali al-Houthi said the spike in attacks on Saudi Arabia has been in response to Saudi aggression in Yemen, including restrictions on the airport in the capital, Sanaa, and the port of Hodeida.

“If the countries of aggression stop attacking us and end the siege on our country, then there will be no attacks on [Saudi Arabia],” he said in a statement to The Washington Post. “This is what we have been proposing from the beginning until today, which is to face the aggression with what would enable us to deter [it].”

Although rebel attacks inside the kingdom have rarely caused significant damage, the Houthis have demonstrated they can carry out more-complex assaults, using “drones at longer ranges accompanied by ballistic missile attacks,” according to Ian Williams, a fellow at the Center for Strategic and International Studies who has researched Houthi missile operations. “Their tactics seem to be getting more sophisticated,” he said. “It has strained [Saudi] defenses.”

He said these advances may actually make it more difficult for the Saudis to extract themselves from the conflict. “It makes it much harder for Saudi to quit Yemen, knowing that you have this hostile force there with these missiles capable of firing into their territory,” Williams said.

In addition to seeking greater leverage in negotiations, the rebels could be carrying out the attacks for their propaganda value, framing them as a reprisal for Saudi airstrikes and making the Saudis appear vulnerable, analysts said. “It’s about embarrassing the Saudis,” said Nadwa Al-Dawsari, a nonresident fellow at the Middle East Institute.

And when the Saudi-led coalition in turn retaliates, the Houthis “can use it to bring attention to the negative intervention of the Saudis and use it to their advantage,” Dawsari said.

Last week, the U.S. Treasury Department imposed sanctions on two Houthi leaders, alleging their involvement in attacks in Saudi Arabia and on commercial vessels.

The Houthis have also escalated their military campaign on the ground. They have intensified an offensive aimed at capturing the strategic city of Marib, which hosts a large number of troops loyal to Saudi-backed president Abed Rabbo Mansour Hadi waging a thus-far futile effort to retake Sanaa.

The fighting in Marib, which has included some of the deadliest clashes in years, threatens to displace hundreds of thousands of Yemenis who had fled violence elsewhere in the country. Marib has so far eluded the Houthis’ grasp.

This escalation came last month after Biden’s team took steps toward removing the Houthis’ terrorist designation, and the offensive has provoked dismay among humanitarian groups and diplomats who had hoped the change at the White House would instead lead to a lowering of tensions.

As Saudi Arabia has come under increasing fire, some questions have emerged over whether the Houthis are responsible for all the attacks they have claimed within the kingdom. The Saudi government said the missile and drone attack earlier this week on an Aramco facility in Ras Tanura, a major port on the Persian Gulf coast, came from the direction of the sea, casting doubt on whether the strikes were launched from Yemen in the south or rather from inside Iran or Iraq.

In an editorial, Saudi Arabia’s Al-Riyadh news outlet, which reflects government positions, alleged that Iran is trying to use aggression against the kingdom as a bargaining chip ahead of likely negotiations with the Biden administration over when and how the United States would rejoin the Iran nuclear deal.

Meanwhile, the Houthis appear to be trying to shore up support for the missile strikes at home. Last week, a member of the Houthi political bureau, Mohammed al-Bukhaiti, called on Yemenis to donate funds via their phones for attacks on Saudi Arabia by “calling the number 180” from certain phone networks.

“Every time you click the call button, you enable the Yemeni forces to click the button that launches missiles and drones,” Bukhaiti tweeted.

Dadouch reported from Beirut. Ali Al-Mujahed in Taiz, Yemen, contributed to this report.