The U.S. withdrew funding to help transport South Sudan rebel leader Riek Machar to the capital, Juba, to take part in a transitional government, the renegade force’s head of foreign relations said.

Machar’s supporters are now seeking support from the United Nations to fly Machar from Gambella, in western Ethiopia, to Juba, Ezekiel Lol Gatkuoth said Sunday in an interview. The rebels are also appealing to the U.S. to reconsider because Machar’s failure to travel may imperil a peace agreement aimed at ending the country’s two-year civil war, he said.

“Definitely the transition is on hold unless we have another person that can volunteer to help transport the first vice president,” Gatkuoth said.

South Sudan President Salva Kiir and Machar signed a peace deal in August under international pressure, agreeing to run a government of national unity and hold elections within 30 months. Fighting that erupted in December 2013 has killed tens of thousands of people, forced more than 2 million more to flee their homes and cut oil production by at least a third.

After the government denied permission for Machar to land on Saturday, there was a “further lack of action” to arrange flights for Monday, the British Embassy in Juba said in a statement on its Facebook page. “It is now time for the parties to take over the primary responsibility of ensuring this return.”

Machar’s scheduled April 18 return has been repeatedly delayed because of disagreements about the number of troops and the amount of weapons he can travel with to Juba.

(Updates with comment from U.K. government in fifth paragraph)