WASHINGTON (Reuters) - The top U.S. diplomat for Africa welcomed a rapprochement between Ethiopia and Eritrea ending two decades of hostility but said concerns over Eritrea’s human rights record hindered cooperation with Washington.

FILE PHOTO: Eritrea's President Isaias Afwerki and Ethiopia's Prime Minister, Abiy Ahmed arrive for an inauguration ceremony marking the reopening of the Eritrean embassy in Addis Ababa, Ethiopia July 16, 2018. REUTERS/Tiksa Negeri//File Photo

The leaders of Ethiopia and Eritrea re-opened crossing points on their shared border for the first time in 20 years on Tuesday, raising hopes of reduced tensions in the region.

Tibor Nagy, the U.S. State Department’s assistant secretary for Africa, told a congressional hearing on Wednesday that the United States had “deliberately engaged” with Eritrea in recent months but it was too soon to talk about lifting United Nations sanctions imposed in 2009, which accused it of supporting Islamist militants in Somalia. Eritrea denied the charge.

Among concerns that the United States had raised with Eritrea was the detention of U.S. embassy local staff and several Americans for what Nagy called politically-motivated reasons.

The United States also wanted a full explanation from Eritrea over past weapons purchases from North Korea highlighted in a U.N. report, said Nagy, without elaborating.

He said the jailing of religious and political prisoners and indefinite, obligatory national service, as well as a tightly-controlled system of government were also a worry.

“Eritrea cannot assume that by saying wonderful things and opening good relations with the neighbors that will automatically lead to sanctions relief,” said Nagy, a former U.S. ambassador to Ethiopia.

“There have to be concrete actions taken and we will remain very engaged and say things that may not always be popular but have to be said,” he added.

Eritrea has long dismissed accusations of human rights abuses by the U.N., including alleged extrajudicial killings and torture, as “totally unfounded and without merit.”

The U.N. imposed sanctions on Eritrea in 2009, backed by 13 of the 15 members of the U.N. Security Council. The sanctions included an arms embargo, travel restrictions and asset freezes for some of the country’s top officials.

But warming ties between Eritrea and Ethiopia this year and sweeping reforms by Ethiopia’s new Prime Minister Abiy Ahmed have reshaped the political landscape in the Horn of Africa.

Abiy’s ruling coalition has ended a state of emergency and released political prisoners, while also announcing plans to partially open up the economy to foreign investors.

In his boldest move, Abiy offered last month to make peace with Eritrea, 20 years after the neighbors started a border war that killed an estimated 80,000 people. Full-blown fighting ended by 2000, but their troops have faced off across their disputed frontier ever since.

“Up to now for the last 20-plus years Eritrea has used Ethiopia as an excuse to maintain what I would almost call a ‘fortress state’,” Nagy said. “With the opening of peace they really will no longer have a reason to do that.”