SANAA, Aug. 31 (Xinhua) -- UN Special Envoy to Yemen Hans Grundberg on Sunday urged de-escalation in Yemen as tensions rose between the Houthi group and Israel after Prime Minister Ahmed al-Rahawi of the Houthi-backed government was killed in Israeli airstrikes on Thursday.

"Yemen cannot afford to become a battleground for a broader geopolitical conflict," Grundberg said in a statement.

"These attacks need to stop," he said, urging all parties to "use available diplomatic channels to de-escalate" and to "protect civilians and civilian infrastructure in line with their obligations under international law."

The Houthis said on Saturday that al-Rahawi and several other ministers were killed in Israeli airstrikes on Sanaa on Thursday while they were attending a government workshop. The group vowed to retaliate, with Mahdi al-Mashat, head of the Supreme Political Council -- the Houthis' top governing body -- warning in a speech aired by Houthi-run al-Masirah TV that "Israel should await dark days."

The Houthis seized Sanaa in 2014, forcing the internationally recognized Yemeni government into exile in Aden. Since then, they have controlled much of northern Yemen, including Sanaa and Hodeidah, and set up an unrecognized government in the capital.

On Sunday, Israeli Prime Minister Benjamin Netanyahu told a weekly cabinet meeting that Israel would eliminate all Houthi leaders.

In a separate development, Grundberg condemned what he called a "new wave" of arbitrary detentions of UN personnel by the Houthis.

At least 11 UN staff members were detained on Sunday in Sanaa and Hodeidah, bringing the total number of detained UN staff in northern Yemen to 34, he said.

Grundberg urged the Houthis to "immediately and unconditionally" release the detainees, warning that the continued detention of UN staff, aid workers, and civil society activists was undermining humanitarian operations and efforts to advance peace in Yemen.

Several employees of the UN World Food Program (WFP) were also arrested by the Houthis in Sanaa on Sunday, security and humanitarian sources told Xinhua.

A local security official, speaking on condition of anonymity, said one WFP employee was taken from the organization's headquarters in Sanaa, while several others were arrested from its office in Hodeidah. A WFP source in Yemen confirmed the arrests, adding that three security guards working at the Sanaa headquarters were also detained when Houthi forces stormed the building.

There has been no immediate comment from the Houthis regarding Sunday's arrests. ■