Sana'a -- Earlier this week, 20 UN staff, including 15 foreigners, were released after being held in their compound since a raid last weekend.

The rebels have harassed and detained UN staff and aid workers for years, accusing them of spying, but they have accelerated arrests since the start of the Gaza war.

"Seven United Nations employees, all of them Yemenis, have been arrested from late last night until this afternoon on charges of spying for Israel," a security source in Sanaa told AFP.

Sana'a -- Another Huthi source confirmed UN employees had been arrested but did not specify how many.

At a regular briefing on Friday, UN spokesperson Farhan Haq said that two of its staff had been arrested the day before -- among the 55 UN personnel currently in Huthi detention.

"These actions are forcing us to reassess the way in which we work in areas controlled by the Huthis," Haq said.

The internationally recognised Yemeni government in Aden -- which opposes the Huthis -- condemned the new arrests, labelling them an escalation.

The Huthis, part of Iran's "axis of resistance" against Israel and the United States, have frequently fired at ships in the Red Sea and at Israeli territory during the two-year Gaza war, claiming solidarity with the Palestinians.

Israel has launched numerous retaliatory strikes, including a major attack in August that killed the Huthis' premier and nearly half of his cabinet.

Earlier this month, rebel leader Abdulmalik al-Huthi accused UN employees of having a hand in the attack without offering evidence. The UN has rejected the claim.

In mid-September, the UN humanitarian coordinator in Yemen was transferred from Sanaa to the interim capital Aden.