Jan 10 (Reuters) - U.S. President Joe Biden raised concerns about air strikes in the conflict in northern Ethiopia and about human rights issues during a call with Ethiopian Prime Minister Abiy Ahmed on Monday, the White House said. Thousands have died and millions have been displaced since war broke out in Ethiopia"s Tigray region in November 2020 between Abiy"s federal forces, backed by regional allies, and the Tigray People"s Liberation Front (TPLF) that governs the region. In a tweet, Abiy described his conversation with Biden as "candid" and said the pair had discussed strengthening cooperation, while a senior U.S. administration official said the call was business-like and focused on issues. "President Biden expressed concern that the ongoing hostilities, including recent air strikes, continue to cause civilian casualties and suffering," the White House said in a statement after the two leaders spoke. An air strike in Tigray killed 56 people in a camp for displaced people and injured 30, including children, two aid workers told Reuters on Saturday. read more Aid organisations suspended their operations in the area of northwest Tigray where the strike took place, the U.N. agency for humanitarian affairs said on Sunday, citing an ongoing threat of drone strikes. U.N. Secretary-General Antonio Guterres in a statement on Monday said the United Nations and its partners were working with authorities to urgently mobilize emergency assistance to the area of a refugee camp in Tigray hit by another air strike last week that killed three Eritrean refugees. read more The senior U.S. official said Abiy has been sending more positive signals in recent days, including through the release of prisoners and openness to dialogue, but warned it is hard to know how long the constructive phase will last. Some TPLF leaders were among opposition leaders freed from prison, the Ethiopian state broadcaster announced on Friday, as the government said it would begin dialogue with political opponents. read more The U.S. official said Biden told Abiy where the United States wants to continue to see progress, including stopping air strikes, expanding humanitarian access to all regions of the country and engaging in ceasefire talks. "We do see this as a moment of opportunity, if the parties are willing and able to seize it. That remains to be seen and this window won"t be open forever," said the official. The White House also said Biden and Abiy discussed "the need to address the human rights concerns of all affected Ethiopians, including concerns about detentions of Ethiopians under the state of emergency." Human rights groups and the United Nations have raised concerns about large-scale detentions of ethnic Tigrayans. Abiy"s government has denied any ethnically motivated detentions were taking place. Abiy"s appointment as prime minister in 2018 ended nearly three decades during which the TPLF, originally a rebel group which came to power in 1991, dominated Ethiopia"s ruling coalition. The TPLF says Abiy wants to end the ethnically-based federal government system they created, while Abiy says the TPLF is hungry to return to national power. Each side rejects the other"s narrative. Ethiopia, Africa"s second most populous nation, is a major power in the Horn of Africa. The United States has long partnered with Ethiopia to try to contain Islamist militants in neighbouring Somalia. But relations have become more tense since the Tigray conflict broke out. Ethiopia previously accused Washington of meddling in its internal affairs. read more The official said Monday"s call was in part intended to put the U.S.-Ethiopia relationship onto constructive footing. The United States recently cut Ethiopia from access to a duty-free trade programme, following through on Biden"s threat to do so over alleged human rights violations in Tigray.