In this Tuesday, April 21, 2015 file photo, a Saudi soldier stands alert, on the Saudi-Yemen border in Najran, Saudi Arabia.

Saudi forces on Monday targeted Yemeni Houthi militiamen across the border after the fighters fired at an army outpost.

The fighting comes as a Saudi-led Arab coalition resumed air strikes on the Iranian-allied Houthi militia positions in and around the southern Yemeni city of Aden overnight after a five-day humanitarian truce expired on Sunday.

Bombings struck the rebel-held presidential palace in Aden, groups of militiamen on the western and eastern approaches to the city as well as the international airport where Houthis and local fighters have been clashing, said residents.

There was no word on any casualties.

Explosions could be heard near the southern city’s airport and the districts of Khor Maksar and Crater shortly after the five-day ceasefire expired on Sunday.

Yemen’s foreign minister told Reuters the Saudi-led coalition had decided not to renew the truce because the agreement had been repeatedly broken by the Houthi militias.

“That’s what we said before — that if they start again, we will start again,” said Reyad Yassin Abdullah from Yemen’s exiled government in Riyadh.

U.S. Secretary of State John Kerry said on Monday Washington supported extending a “humanitarian pause” in the fighting in Yemen, but that maneuvers by Houthi militias made that difficult.

“We know that the Houthis were engaged in moving some missile-launching capacity to the border and under the rules of engagement, it was always understood that if there were proactive moves by one side or another, then that would be in violation of the ceasefire agreement,” Kerry told reporters in the South Korean capital.

The Houthi militias were not immediately available for comment.

Saudi Arabia and its Arab allies have been conducting an offensive against the Houthis and units loyal to former president Ali Abdullah Saleh for more than six weeks, saying the rebels are backed by Iran.

The campaign has yet to reverse the Houthis’ advance into Aden and along battlefronts across Yemen’s south.

A five-day truce that started on Tuesday night halted the air strikes and allowed humanitarian aid into the blockaded country, though residents of the remote southern provinces of Shabwa, Dhalea and Abyan said heavy ground fighting persisted despite the agreed pause.

Since Tuesday, Saudi-led forces and the Houthi militias had largely observed the ceasefire meant to allow delivery of food, fuel and medical supplies to millions of Yemenis caught in the conflict since the alliance began air strikes on March 26.

The coalition was not considering any new ceasefire according to Yemen’s foreign minister Reyad Yassin Abdullah.

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