Secretary-General António Guterres said it was engaging with the government “in the expectation” the affected staff could “continue their important work”.
Ethiopia earlier declared the seven “persona non grata” and said they had 72 hours to leave the country.
The UN has raised concerns in recent weeks about a “de-facto” blockade of aid to the war-torn region of Tigray.
UN aid chief Martin Griffiths earlier this week said he assumed there was now famine in Tigray and urged the Ethiopian government to “get those trucks moving”.
Ethiopia’s UN mission in New York said claims of a blockade were “baseless”.
Thousands of people have been killed and more than two million have fled their homes since Prime Minister Abiy Ahmed ordered a military offensive against regional forces in Tigray in November 2020.
He said he did so in response to an attack on a military base housing government troops there.
The escalation came after months of feuding between Mr Abiy’s government and the TPLF over the reforms he was pursuing.
Ethiopia has declared the TPLF a terrorist organisation, while it insists that it is the legitimate government in Tigray.
The expulsions announced on Thursday include the head of the UN Office for the Coordination of Humanitarian Affairs (UNOCHA) and the head of the UN Children’s Fund (Unicef) in Ethiopia. It is not clear what the allegations are against them.
Washington has condemned “in the strongest possible terms” Ethiopia’s “unprecedented” decision.
White House Press Secretary Jen Psaki said the US “will not hesitate” to impose sanctions against “those who obstruct humanitarian assistance”.
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Mr Guterres, in his statement, said “the UN is delivering lifesaving aid – including food, medicine, water, and sanitation supplies – to people in desperate need.
“I have full confidence in the UN staff who are in Ethiopia doing this work,” he added.
The Ethiopian government and its aid partners acknowledge there is a crisis in Tigray and have previously made efforts to work together to get relief into the region, the BBC’s Africa correspondent Catherine Byaruhanga notes.
But the decision to expel senior UN officials shows just how bad the relationship is between both sides, and how much more diplomatic work needs to be done to get emergency assistance to those in need, she adds.
The UN said earlier this month that of 466 trucks that entered Tigray between mid-July and mid-September, only 38 had made the return journey.
Both the Ethiopian government and the Tigray People’s Liberation Front (TPLF), which controls the northern region, blame the other for this.
The TPLF say truck drivers are provided with only enough fuel for a one-way journey into Tigray and complain of violence and intimidation at checkpoints manned by the Ethiopian federal forces.
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The Ethiopian government has rejected the suggestion that a shortage of fuel is stopping the trucks. It has also accused aid organisations of supporting the TPLF.
Mr Griffiths told Reuters earlier this week that a UN assessment in June had predicted there were 400,000 people in famine-like conditions “and the supposition was that if no aid got to them adequately they would slip into famine.”
“I have to assume that something like that is happening,” he said.