U.S. lawmakers have urged President Barak Obama to pressure the United Nations to impose arms embargo on South Sudan after the heavy fighting in Juba in July.

A draft U.N. Security Council resolution calls for a vote on an arms embargo if the U.N. Secretary-General reports that South Sudan’s authorities have blocked deployment of a regional force to help restore calm.

The letter to Obama, obtained by The Associated Press, says the lack of an arms embargo would pose a direct threat to that regional force. It says both sides in the fighting have attacked civilians, aid workers and diplomats.

Human Rights Watch has accused Washington of failing to adequately push for an arms embargo at the Security Council, where the U.S. leads South Sudan policy.

However, the government of South Sudan says it rejects the U.S. proposal to the U.N. for sending 4,000 strong foreign force to South Sudan, and also rejects any attempt for the imposition of an arms embargo on the country.