WASHINGTON, Aug. 9 (Xinhua) -- As accusations grew over U.S. reported support of the Saudi-led coalition in the recent attack in Yemen that had caused huge civilian casualties, U.S. State Department on Thursday urged to conduct a thorough and transparent investigation into the airstrikes.

In a press briefing, State Department spokesperson Heather Nauert said although Washington does not have full details of the attacks on the ground, it is certainly "concerned about" the reports of civilian casualties in the war-torn Middle East nation.

"We call on the Saudi-led coalition to conduct a thorough and transparent investigation into the incident. We take all credible accounts of civilian casualties very seriously. We call on the parties to take appropriate measures to protect civilians in accordance with international law and urge all parties to investigate all reported incidents of civilian casualties," she said.

Washington has been reportedly provided weapons, training and targeting data to Saudi Arabia that had facilitated the attacks.

As questions has also mounted over the Saudi-led coalition's earlier assertion that the attacks had been conducted in accordance with international law, the call for further investigation into the issue by independent bodies has been rising.

Nauert dodged the questions, only saying that "we regret any loss of civilian life" and "Saudi Arabia is an important strategic partner in the region to the United States."

At least 43 civilians, mostly children, were killed on Thursday when Saudi-led coalition air strikes hit buses in Yemen's northern province of Saada.

The Saudi-led coalition later defended the airstrike as "legitimate" response to a deadly Houthi attack on the Saudi border city Jazan.

Houthi spokesman Mohammed Abdulsalam mocked the statement, saying "this is a war crime."

Henrietta Fore, head of the United Nations Children's Fund (UNICEF), strongly condemned the airstrike.

"Attacks on children are absolutely unacceptable," said Fore. "I'm horrified by the reported airstrike on innocent children, some with UNICEF backpacks. Enough is enough."

"Attacking children is the lowest any party of this conflict can go," UNICEF Yemen Resident Representative Meritxell Relano told UN News. "There is no justification whatsoever to attacking children."

The Saada attack was the latest in a series of recent airstrikes against civilians launched by the Saudi-led coalition on Yemen.

Last week, the Saudi-led coalition airstrikes struck the gate of al-Thawra hospital and adjacent fish market in Yemen's Red Sea port city of Hodeidah, killing 52 civilians and wounding 102 others.

Saudi Arabia has led an Arab military coalition that had intervened in the Yemen war since 2015 to support the government of exiled President Abd-Rabbu Mansour Hadi.