Thirteen civilians including two children were killed in a Saudi artillery strike in northern Yemen on Monday in response to a rebel attack on a Saudi base, medics said.

They said the strike on the Al-Thabit Al-Shaabi district of Saada province, a stronghold of the Huthi rebel group, also wounded 26 people including 12 children.

Rebel-controlled media including the Huthis’ Al-Masirah channel earlier reported Saudi artillery fire on a market in the same area.

The channel reported 36 casualties, including both dead and wounded, while the rebels’ spokesman said at least 10 civilians had died.

Rebel media reported that the Huthis had carried out a drone attack against the King Khaled airbase in the Saudi city of Khamis Mushait.

Saudi authorities gave no immediate confirmation of the drone attack or the artillery fire.

Saudi Arabia heads a military coalition that has since 2015 been battling the Huthis, which are backed by the kingdom’s regional arch-rival Iran.

The Huthi rebels, who have faced persistent and deadly coalition bombing since March 2015 that has exacted a heavy civilian death toll, have stepped up missile and drone attacks across the border.

In June alone, they launched at least 20 missile and drone attacks on the oil-rich kingdom, challenging Saudi Arabia’s advanced defense technology.

Saudi air defenses intercepted a Yemeni rebel drone targeting a southwestern city last Thursday.

Nine civilians were wounded in a July 3 Huthi attack on Abha airport in the south of the kingdom, the coalition said.

A June 12 missile attack on the same airport wounded 26 civilians, drawing warnings of “stern action” from the coalition.

And on June 23, a rebel attack on Abha airport killed a Syrian and wounded 21 other civilians, the coalition said.

The uptick in cross-border raids comes amid heightened regional tensions as key Saudi ally the United States presses a “maximum pressure” campaign against its arch-rival Iran after withdrawing from a landmark 2015 nuclear deal between major powers and Tehran.

Saudi Arabia has repeatedly accused Iran of supplying sophisticated weapons to the Huthis, a charge Tehran denies.

The coalition intervened in support of the Yemeni government in 2015 when President Abedrabbo Mansour Hadi fled into Saudi exile as the rebels closed in on his last remaining territory in and around second city Aden.

Yemen’s conflict has claimed tens of thousands of lives, according to relief agencies, and triggered what the United Nations describes as the world’s worst humanitarian crisis.

– Agence France-Presse