Smoke rises from Al-Qahira castle, an ancient fortress that was recently taken over by Houthis, following a Saudi-led airstrike in Taez city, Yemen, Tuesday, May 12, 2015.

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.

A Saudi-led coalition resumed air strikes against the Houthi in Aden overnight, hours after the expiry of a truce meant to facilitate badly needed humanitarian aid, a Reuters witness said.

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.

“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.

“Saudi Arabia, under the rules of engagement, took action. We continue to support the idea of extending the humanitarian pause, but I think under the circumstances at the moment that would be difficult.”

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.

Sporadic clashes had continued, however, with at least 15 killed overnight Saturday-Sunday in the cities of Taiz and Dhalea, residents said.

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