WASHINGTON, July 29 (Reuters) – Eritrea has only a short time to stop undermining security in Somalia or face possible U.N. sanctions, Washington’s ambassador to the United Nations, Susan Rice, said on Wednesday.

Rice told a congressional committee the United States was “deeply concerned and very frustrated” with Eritrea’s behavior in Somalia, including arming and funding Islamist insurgents

“It is unacceptable, and we will not tolerate it, and nor will other members of the Security Council,” she told the House Foreign Affairs Committee.

The U.N. Security Council warned Eritrea this month it would consider action against anyone undermining peace in Somalia.

“We will continue to discuss with colleagues in the Security Council, appropriate measures including potentially sanctions, against Eritrea for its actions in Somalia,” Rice told the committee.

“There is a very short window for Eritrea to signal through its actions that it wishes a better relationship with the United States and indeed the wider international community.

“If we do not see signs of that signal in short order, I can assure you that we will be taking appropriate steps with partners in Africa and the Security Council,” she said.

Somalia’s government and others have accused Eritrea of supplying arms to insurgents in breach of a U.N. embargo that allows such shipments only to the government.

The African Union, which has a force of 4,300 peacekeepers in Somalia, has called on the United Nations to impose sanctions on Eritrea for backing the rebels.

Eritrean officials deny the charges of arms supplies.

Al Qaeda-linked fighters belonging to the al Shabaab insurgent group control much of southern and central Somalia and most of the capital Mogadishu.

Rice said the Eritreans had rebuffed repeated U.N. attempts to discuss the situation. She said that Eritrea had essentially “stiffed and stonewalled” the U.N. (Editing by Alan Elsner)