Officials in rebel-controlled areas of Yemen have halted plans to impose a 2 percent tax on humanitarian assistance, after the measure sparked an outcry from donor countries and aid groups that said they might be required to curtail needed aid.

The Houthi rebel-controlled government said in a letter to Mark Lowcock, the U.N. undersecretary general for humanitarian affairs, that the fee had been intended “only to cover the basic necessary costs so we can put forward all the help and facilitation for partners in humanitarian work.”

But the Feb. 11 letter said the Houthis’ aid council had decided “to suspend use of the 2 percent and to not implement it in 2020 on the principle of finding alternate solutions.”

The proposed tax, along with other restrictions, has prompted organizations and donor nations to consider changing the way they work in Yemen, where a prolonged conflict between the Iranian-linked Houthis and a rival Saudi-backed Yemeni government has resulted in the world’s worst humanitarian crisis.

How to deliver aid to the millions of Yemenis in need of medical care and sustenance was the chief topic when officials from aid groups, donor nations and the United Nations met in Brussels this week. Humanitarian groups say problems in Houthi areas have mounted since last year, including delays in securing approval for projects and travel permits for aid workers, as well as harassment of aid officials.

The situation has deepened fears about the diversion of assistance to the Houthis’ military activities. U.S. officials said earlier this week they were considering a suspension of assistance to Houthi-controlled areas, a step that would have a major impact given the United States’ status as one of the top two donors to Yemen.

Aid workers said the 2 percent tax was particularly problematic because it would violate their fiduciary responsibility to funders.

It is not clear what other “alternate solutions” the Houthi government might pursue in lieu of the 2 percent tax and whether aid groups would still consider suspending aid based on other restrictions.

The State Department did not provide an immediate comment on the Houthi announcement.

After the discussion in Brussels on Thursday, aid officials said they had jointly decided to recalibrate, reduce or even suspend certain aid “if and where principled delivery is impossible.”

“Assistance to vulnerable people will continue as long as they can be reached in line with humanitarian principles and with full accountability,” aid officials said in a summary of those talks.

Speaking on the sidelines of a security conference in Munich, Jan Egeland, secretary general of the Norwegian Refugee Council, said “a lot of things” remained problematic.

“We just want them to adopt the rules for humanitarian work worldwide and in conflict areas for a generation,” he said.

Mark Green, the administrator for the U.S. Agency for International Development, who is also in Munich, told reporters the Trump administration was working with other donors and aid groups but that “we’re not willing to allow our systems to be diverted.”

Al-Mujahed reported from Sanaa, Yemen, and Hudson reported from Munich.