SANAA — Saudi-led forces said on Thursday they would respond harshly against Yemen’s Houthi rebels following attacks on citizens in border areas, hours after the kingdom offered a five-day humanitarian truce if the militants stopped fighting.

Brigadier General Ahmed Asseri, spokesman for the Saudi-led coalition, said the Houthis had changed the situation by targeting cities in recent days, and vowed that the coalition would go after leaders of the rebel group.

“The Houthis are now targeting the borders of the kingdom and the situation is that we will defend our citizens,” Asseri said, adding that 15 people were wounded in Houthi shelling on the city of Najran on Thursday evening.

Asseri said Saudi-led forces would keep all options open, but declined to say if a ground offensive was being planned.

The remarks came hours after Saudi Foreign Minister Adel al-Jubeir announced at a joint news conference with  Secretary of State John Kerry that the kingdom was ready to offer a five-day humanitarian ceasefire if the Houthis honored the truce.

“The pause will affect all of Yemen for a period of five days,” Saudi foreign minister Adel al-Jubeir said. “The actual date will be announced shortly as well as the requirements. This is all based on the Houthis complying with the ceasefire.”

International concern about Yemen’s dire humanitarian situation has grown as fighting, air strikes and an arms embargo have caused civilian deaths, internal displacement, destruction of infrastructure and shortages of food, medicine and fuel.

The offer of a truce came days after the Houthis had shelled Saudi border towns, prompting renewed coalition air strikes in Yemen, and as the rebels advanced into a last central area of Aden, a city whose fate is seen as pivotal to Yemen’s civil war.

Kerry welcomed the Saudi truce offer and said neither Riyadh nor Washington was talking about sending ground troops to Yemen.

On Thursday, Houthi-linked al-Masirah TV showed fighters in a key district of Aden, and reported that the presidential palace was in their hands.

In a letter to the United Nations, the Houthis called for international action to stop what they described as Saudi aggression and crimes against the Yemeni people.

Villagers from Yemen’s northern Hajah province say many residents have left their homes because of the Saudi bombardment and are seeking refuge in other areas where they lack food.

The Saudi-led coalition began strikes against the Houthis and forces loyal to ex-president Ali Abdullah Saleh on March 26 with the aim of restoring the government of President Abd-Rabbu Mansour Hadi.

Riyadh fears the Houthis, who are allied to its foe Iran, will dominate Yemen.